chore(rules): enforce API documentation verification in PLAN phase

packages/rules/.ai-rules/rules/core.md

@@ -94,6 +94,18 @@ feat(auth): add token validation with session management
 **Purpose:**
 Create actionable implementation plans following TDD and augmented coding principles
 
+### API Assumption Verification (PLAN mode)
+
+When a plan references external API fields, schemas, or behaviors:
+1. **Cite source**: Include the official documentation URL for every API assumption
+2. **Mark unverified**: Tag any assumption without a verified source as `[UNVERIFIED]`
+3. **Verify before ACT**: All `[UNVERIFIED]` items must be verified (via WebFetch/WebSearch) before transitioning to ACT mode
+4. **No fabricated schemas**: Never assume API input/output fields exist without documentation proof
+
+Example:
+- ✅ "Hook stdin includes `tool_name` field ([source](https://code.claude.com/docs/en/hooks))"
+- ❌ "Hook stdin includes `session_cost` field" (no source → will cause implementation failure)
+
 ---
 
 ### Structured Reasoning Process (SRP)

packages/rules/.ai-rules/skills/writing-plans/SKILL.md

@@ -92,6 +92,24 @@ Every non-trivial decision in the plan MUST include at least two approaches with
 - State the chosen approach with a clear rationale
 - If only one viable approach exists, explain why alternatives were ruled out
 
+## API Verification
+
+When the plan references external APIs, SDKs, or protocols:
+
+1. **Document every API assumption** with its source URL
+2. **Create an "API Assumptions" table** in the plan:
+
+| Assumption | Verified? | Source |
+|-----------|:---------:|--------|
+| Hook stdin has `tool_name` | ✅ | [Hooks Reference](url) |
+| Hook stdin has `session_cost` | ❌ UNVERIFIED | — |
+
+1. **All assumptions must be verified before ACT phase**
+2. **WebFetch/WebSearch the official docs** — never rely on memory or guesses
+
+### Why This Matters
+Unverified API assumptions propagate into implementation, causing features to be unimplementable as designed. Verification at PLAN time costs minutes; rework at ACT time costs hours.
+
 ## Task Structure
 
 ```markdown