feat(actions-fleet): install planner with unified diff preview

[Copilot]

Adds a readable diff preview to the install planner so reviewers can inspect exact file additions, modifications, and conflicts before any writes occur.

Core library (actions-fleet-core)

• PlannedFileDiff — adds existingContent: string | null field; the on-disk content is now captured during planDiff and carried through for diffing
• render-diff.ts (new) — zero-dependency LCS-based diff engine:
  • renderUnifiedDiff(old, new, filename, context?) — produces standard diff -u output; null old content renders as all-additions (new file)
  • renderPlanPreview(plan, context?) — full plan preview: additions for create, unified diff for update-managed, # CONFLICT warning headers with proposed content for conflict-unmanaged / conflict-other-pack, unchanged files omitted

CLI (sh1pt actions plan)

New --diff flag prints a colorized unified diff after the file status table:

$ sh1pt actions plan node-pnpm-ci --diff

Plan: node-pnpm-ci@1.0.0 → /my/repo

  create               .github/workflows/ci.yml

Diff preview:

--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -1,4 +1,4 @@
+name: CI
+on: [push, pull_request]
...

---/+++ headers bold · - lines red · + lines green · @@ hunks cyan · # CONFLICT warnings yellow.

[alwaysmeticulous]

Meticulous was unable to execute a test run for this PR because the most recent commit is associated with multiple PRs. To execute a test run, please try pushing up a new commit that is only associated with this PR.

_{Last updated for commit f505900. This comment will update as new commits are pushed.}