Add plan references with keyed identifiers

app/admin/references.rb

@@ -0,0 +1,32 @@
+ActiveAdmin.register CoPlan::Reference, as: "Reference" do
+  permit_params :plan_id, :key, :url, :title, :reference_type, :source, :target_plan_id
+
+  index do
+    selectable_column
+    id_column
+    column :plan
+    column :key
+    column :url
+    column :title
+    column :reference_type
+    column :source
+    column :target_plan_id
+    column :created_at
+    actions
+  end
+
+  show do
+    attributes_table do
+      row :id
+      row :plan
+      row :key
+      row :url
+      row :title
+      row :reference_type
+      row :source
+      row :target_plan_id
+      row :created_at
+      row :updated_at
+    end
+  end
+end

db/migrate/20260410182145_create_coplan_references.co_plan.rb

@@ -0,0 +1,22 @@
+# This migration comes from co_plan (originally 20260410000000)
+class CreateCoplanReferences < ActiveRecord::Migration[8.1]
+  def change
+    create_table :coplan_references, id: { type: :string, limit: 36 } do |t|
+      t.string :plan_id, limit: 36, null: false
+      t.string :key
+      t.string :url, null: false
+      t.string :title
+      t.string :reference_type, null: false
+      t.string :source, null: false
+      t.string :target_plan_id, limit: 36
+      t.timestamps
+    end
+
+    add_index :coplan_references, [:plan_id, :key], unique: true
+    add_index :coplan_references, [:plan_id, :url], unique: true
+    add_index :coplan_references, :target_plan_id
+    add_index :coplan_references, :source
+    add_foreign_key :coplan_references, :coplan_plans, column: :plan_id
+    add_foreign_key :coplan_references, :coplan_plans, column: :target_plan_id
+  end
+end

engine/app/assets/stylesheets/coplan/application.css

@@ -1872,6 +1872,193 @@ body:has(.comment-toolbar) .main-content {
   margin-top: var(--space-sm);
 }
 
+/* Plan tabs */
+.plan-tabs__nav {
+  display: flex;
+  gap: var(--space-xs);
+  margin-bottom: calc(-1px);
+  position: relative;
+  z-index: 1;
+}
+
+.plan-tabs__tab {
+  padding: var(--space-sm) var(--space-lg);
+  font-size: var(--text-sm);
+  font-weight: 500;
+  color: var(--color-text-muted);
+  text-decoration: none;
+  border: 1px solid transparent;
+  border-bottom: none;
+  border-radius: var(--radius) var(--radius) 0 0;
+  transition: color 0.15s, background 0.15s;
+  cursor: pointer;
+}
+
+.plan-tabs__tab:hover {
+  color: var(--color-text);
+  background: var(--color-bg);
+  text-decoration: none;
+}
+
+.plan-tabs__tab--active {
+  color: var(--color-text);
+  background: var(--color-surface);
+  border-color: var(--color-border);
+  font-weight: 600;
+}
+
+.plan-tabs__count {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  font-weight: 400;
+}
+
+.plan-tabs__panel--hidden {
+  display: none;
+}
+
+/* References section */
+.references-section {
+  padding: var(--space-lg);
+}
+
+.references-list {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+}
+
+.references-list__item {
+  display: flex;
+  align-items: flex-start;
+  gap: var(--space-sm);
+  padding: var(--space-sm) 0;
+  border-bottom: 1px solid var(--color-border);
+}
+
+.references-list__item:last-child {
+  border-bottom: none;
+}
+
+.references-list__icon {
+  flex-shrink: 0;
+  font-size: var(--text-lg);
+  line-height: 1.4;
+}
+
+.references-list__content {
+  flex: 1;
+  min-width: 0;
+}
+
+.references-list__link {
+  display: block;
+  word-break: break-all;
+  font-size: var(--text-sm);
+}
+
+.references-list__meta {
+  display: flex;
+  align-items: center;
+  gap: var(--space-xs);
+  margin-top: 2px;
+}
+
+.references-list__source {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  font-style: italic;
+}
+
+.badge--ref-type {
+  font-size: 0.7rem;
+  padding: 1px 6px;
+  border-radius: 4px;
+  background: var(--color-bg);
+  color: var(--color-text-muted);
+  border: 1px solid var(--color-border);
+}
+
+.badge--ref-key {
+  font-size: 0.7rem;
+  padding: 1px 6px;
+  border-radius: 4px;
+  background: var(--color-surface);
+  color: var(--color-primary);
+  border: 1px solid var(--color-primary);
+  font-family: var(--font-mono, monospace);
+}
+
+.references-list__remove {
+  flex-shrink: 0;
+  background: none;
+  border: none;
+  color: var(--color-text-muted);
+  cursor: pointer;
+  padding: 2px 6px;
+  font-size: var(--text-sm);
+  border-radius: var(--radius);
+  transition: color 0.15s, background 0.15s;
+}
+
+.references-list__remove:hover {
+  color: var(--color-danger);
+  background: var(--color-bg);
+}
+
+.references-section__header {
+  margin-bottom: var(--space-md);
+}
+
+.references-add summary {
+  cursor: pointer;
+  list-style: none;
+}
+
+.references-add summary::-webkit-details-marker {
+  display: none;
+}
+
+.references-add__form {
+  margin-top: var(--space-sm);
+  display: flex;
+  align-items: flex-end;
+  gap: var(--space-xs);
+  flex-wrap: wrap;
+}
+
+.references-add__form .form-group {
+  margin-bottom: 0;
+  flex: 1;
+  min-width: 140px;
+}
+
+.references-add__form input[type="text"] {
+  width: 100%;
+  padding: 6px var(--space-sm);
+  font-size: var(--text-sm);
+  font-family: var(--font-sans);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius);
+  background: var(--color-input-bg);
+  color: var(--color-text);
+  transition: border-color 0.15s;
+}
+
+.references-add__form input[type="text"]:focus {
+  outline: none;
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 3px var(--color-focus-ring);
+}
+
+.references-add__form input[type="text"]::placeholder {
+  color: var(--color-text-muted);
+}
+
+.references-add__form .btn {
+  flex-shrink: 0;
+}
+
 /* Hide sidebar on small screens */
 @media (max-width: 1024px) {
   .content-nav {

engine/app/controllers/coplan/api/v1/plans_controller.rb

@@ -18,7 +18,18 @@ def index
         def show
           render json: plan_json(@plan).merge(
             current_content: @plan.current_content,
-            current_revision: @plan.current_revision
+            current_revision: @plan.current_revision,
+            references: @plan.references.map { |r|
+              {
+                id: r.id,
+                key: r.key,
+                url: r.url,
+                title: r.title,
+                reference_type: r.reference_type,
+                source: r.source,
+                target_plan_id: r.target_plan_id
+              }
+            }
           )
         end
 
@@ -33,12 +44,26 @@ def create
             end
           end
 
-          plan = Plans::Create.call(
-            title: params[:title],
-            content: params[:content] || "",
-            user: current_user,
-            plan_type_id: plan_type&.id
-          )
+          plan = nil
+          ActiveRecord::Base.transaction do
+            plan = Plans::Create.call(
+              title: params[:title],
+              content: params[:content] || "",
+              user: current_user,
+              plan_type_id: plan_type&.id
+            )
+
+            if params[:references].is_a?(Array)
+              params[:references].each do |ref_params|
+                next unless ref_params[:url].present?
+                ref_type = ref_params[:reference_type].presence || Reference.classify_url(ref_params[:url])
+                ref = plan.references.find_or_initialize_by(url: ref_params[:url])
+                ref.assign_attributes(key: ref_params[:key], title: ref_params[:title], reference_type: ref_type, source: "explicit")
+                ref.save!
+              end
+            end
+          end
+
           render json: plan_json(plan).merge(
             current_content: plan.current_content,
             current_revision: plan.current_revision
@@ -68,6 +93,16 @@ def update
             Plans::TriggerAutomatedReviews.call(plan: @plan, new_status: permitted[:status], triggered_by: current_user)
           end
 
+          if params[:references].is_a?(Array)
+            params[:references].each do |ref_params|
+              next unless ref_params[:url].present?
+              ref_type = ref_params[:reference_type].presence || Reference.classify_url(ref_params[:url])
+              ref = @plan.references.find_or_initialize_by(url: ref_params[:url])
+              ref.assign_attributes(key: ref_params[:key], title: ref_params[:title], reference_type: ref_type, source: "explicit")
+              ref.save!
+            end
+          end
+
           render json: plan_json(@plan).merge(
             current_content: @plan.current_content,
             current_revision: @plan.current_revision