docs: Shift planning guidance from "follow patterns" to "evaluate patterns"

[andrewemark]

Summary

Reframes the plugin's stance on existing codebase patterns. The previous guidance ("Follow existing codebase patterns -- ask the user before deviating from established conventions") treated any precedent as authority, which trained the model to launder tech debt by citing it as a constraint. The new framing requires evaluation: match patterns that are sound, deviate from ones that aren't, and surface unsure calls to the user.

What changed

• CLAUDE.md Behavioral Rule — rewritten from "Follow existing codebase patterns" to "Evaluate existing patterns -- don't reflexively conform."
• planning-guidance/SKILL.md — six surgical edits:
  • New "Evaluate, Don't Just Catalog" subsection in Step 4 (gathering context) with explicit evaluation prompts.
  • Step 4.5 (Confirm Approach) — Architecture bullet now requires match/deviate classification with reasoning, not just "patterns to follow."
  • Plan template ## Architecture Fit section restructured into Match / Match-with-adjustment / Deviate classification.
  • Constraints goodness table — added a guard paragraph: precedent-citing constraints only count as rigorous when the precedent has been evaluated.
  • Step 6 self-review prompt — asks reviewers to assess whether matching is the right call, not just whether patterns are missing.

Why

The plugin had no counter-pressure against propagating bad code. Where a codebase has over-mocked tests, defensive null-guards, leaky abstractions, or scaffolding that should have been deleted, the planning skill elevated them to constraints and propagated them forward. Pattern-following had become pattern-laundering.

Not changed

• research-guidance — discovery framing is appropriate there; adoption decisions belong in planning.
• Driver MCP description language ("returns architecture, key files, conventions") — accurate description, not prescriptive.
• implementation-guidance — inherits from the plan's Architecture Fit section, so the upstream fix carries through.