feat: post-compact re-anchors on conventions via /recall pointer

[bborbe]

Post-compact restores state (carry-over items, watchers, next actions) but not rule-awareness — the summarized context after /compact is exactly the drift trigger /recall exists for.

Adds an optional one-line pointer after the re-arm step:

• 📌 Conventions: run /recall if you feel drifted — re-reads global + project CLAUDE.md.
• Full doc re-read deliberately skipped — the harness re-injects CLAUDE.md into context each turn, so a full recall mostly re-emphasizes rather than recovers.
• Skipped entirely when drift risk is low (nothing touched git/worktrees/repo conventions).

Follow-up task: [[Post-Compact Restores State but Not Rule-Awareness]]

[ben-s-pull-request-reviewer]

{
  "verdict": "approve",
  "summary": "PR adds a documentation-only change: a new optional section in `commands/post-compact.md` guiding operators to emit a `/recall` pointer after re-arm, and a corresponding CHANGELOG entry with the correct `feat:` prefix. No code, no violations found.",
  "comments": [],
  "concerns_addressed": [
    "correctness: drift-risk evaluation — the 'optional' step is purely instructional; the conditional skip logic is not implemented in this PR, so there is nothing to review for edge cases. The concern was about runtime code that does not exist here."
  ]
}