Improve code-review-assist skill from review sessions

[jerm-dro]

Summary

• Review sessions on March 20 and 23 revealed recurring gaps in the code-review-assist skill: missing leaky abstractions, not flagging in-place mutations, inconsistent comment severity, and suggesting non-idiomatic code patterns. This codifies those learnings into the skill so future sessions benefit immediately.
• Add abstraction integrity check to Key Review Questions — flags interface methods that only work for one implementation, type casts on interfaces, and other leaky abstraction signals
• Add mutation of shared state check — flags in-place mutation of long-lived or shared data structures and suggests immutable alternatives
• Add Comment Format section requiring conventional comments (blocker:, suggestion:, nitpick:, question:) with severity calibration guidance
• Add Code Suggestions section requiring .claude/rules/ lookup before suggesting code and concrete examples for external contributors

Type of change

• Other (describe): Claude Code skill improvement

Test plan

• Manual testing (describe below)

Validated against three PRs reviewed in the March 23 session (#4236, #4313, #4318) where these gaps were identified. Each addition directly addresses a documented miss in review-session-notes.md.

Does this introduce a user-facing change?

No

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.81%. Comparing base (efb15e9) to head (5c333eb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4321      +/-   ##
==========================================
- Coverage   68.83%   68.81%   -0.03%     
==========================================
  Files         478      478              
  Lines       48288    48306      +18     
==========================================
  Hits        33241    33241              
- Misses      12343    12348       +5     
- Partials     2704     2717      +13     

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