feat: add planning and code-comment preferences, tighten breaking-change rule

[gsdevme]

Summary

Three preference updates:

• planning.md (new): plans written to local paths (~/.claude/, /tmp, …) are session-only context and must never be referenced from PRs, commits, or code comments; load-bearing plans go in the repo under docs/plans/ where they can be linked durably.
• code-comments.md (new): favour file/class/function/method documentation blocks over inline comments, keep code bodies lean (no narration, no change-history notes, no commented-out code), with YAML and other comment-only config formats exempt.
• commits.md: breaking changes are now marked with a BREAKING CHANGE: footer only — the ! type/scope marker is no longer allowed. The !-style example was replaced with a footer-style one.

Each change is wired into ~/.claude/CLAUDE.md via the repo's documented @-import mechanism (the import lines live outside this repo, per the README).

Test plan

• Confirmed ~/.claude/CLAUDE.md imports each topic file by absolute path and the files load in a new session.

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