Document safe manual branch cleanup policy

[Paladin173]

What changed

• Make merged-branch cleanup best-effort and evidence-based instead of unconditional.
• Accept either Git ancestry or an exact merged pull-request head ref/SHA as proof, covering squash merges whose source commits are not ancestors of main.
• Preserve worktree-bound, advanced, unmerged, or uncertain branches unless deletion is explicitly approved.
• Clarify that GitHub's Admin-only automatic merged-branch deletion setting is optional and does not block completion.

This documents the policy decision recorded in #106 while retaining safe manual cleanup when possible.

Testing

• python tools/check_markdown.py - 8 Markdown files checked, 0 problems.
• git diff --check - passed.

Related #106.

[Paladin173]

Exact-head review

Independent read-only review compared head d35fea7f864913be4a5be9b13bc429e42da9870b with base 5c6d10dcc84ceea3996912b56077e05f3e5f254d.

• No findings.
• The diff is limited to AGENTS.md with three additions and one deletion.
• The policy covers ancestry merges, exact merged pull-request head ref/SHA verification for squash merges, worktree protection, branches advanced after review, and uncertain state.
• GitHub automatic merged-branch deletion is optional; safe manual cleanup remains best-effort.
• All applicable CI checks passed. The path-filtered Android instrumentation job skipped as intended for a documentation-only change.
• Regression RED/GREEN: NOT APPLICABLE because this changes agent guidance only and has no runtime behavior.

Verdict: SAFE TO MERGE.