fix: preflight configured reviewers before Slack admission

[ruby-dlee]

Intent

PR 395 is not delivery-ready until the live high-severity configured-reviewer Slack admission finding is fixed with executable behavior coverage, bounded validation runs through non-skipped test and lint, and current exact-head Crosscheck CLEAR is obtained before autonomous merge. Slack must preflight a nonempty valid configured Crosscheck reviewer roster after allowlisted live-head resolution but before request metering, start acknowledgement, reaction, or review execution; missing/invalid roster must visibly refuse without consuming request capacity or starting review. Authorship, model/account origin, branch, worktree, checkout lineage, task metadata, and launch records remain non-gating historical inputs. Preserve live non-draft merge requirements, exact-head CLEAR, CI/currentness, and atomic expected-head merge.

What Changed

• Preflight a nonempty configured Crosscheck reviewer roster after live-head resolution and refuse invalid rosters before metering, reactions, acknowledgements, or review execution.
• Remove authorship, account, model, branch, worktree, checkout, task, and launch provenance from Crosscheck admission and reviewer selection.
• Atomically record live PR identity during registration while retaining generation protection and exact-head Crosscheck merge gating.

Risk Assessment

⚠️ Medium: The change is a broad semantic policy shift that removes authorship gating, but the new reviewer-roster Slack preflight is placed before metering/reaction/start/review and I did not find a concrete source-verifiable regression.

Testing

The configured baseline command had already passed; I then ran the focused Slack behavior suite and an end-to-end Slack admission exercise showing a missing configured reviewer is visibly refused before metering/reaction/review startup, while a valid configured reviewer proceeds to exact-head CLEAR.

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

⚠️ **Review** - medium risk

✅ No issues found.

✅ **Test** - passed

✅ No issues found.

• if [ "${FM_AZURE_VALIDATION_CELL:-0}" = 1 ]; then exec "$FM_AZURE_VALIDATION_SHARD_BRIDGE" behavior --count "${FM_AZURE_VALIDATION_SHARD_COUNT:-8}"; else exec bin/fm-no-mistakes-test-command.sh; fi
• tests/fm-crosscheck-slack.test.sh
• python3 /tmp/no-mistakes-worker-evidence-1117159427/slack_admission_e2e.py

✅ **Document** - passed

✅ No issues found.

✅ **Lint** - passed

✅ No issues found.

✅ **Push** - passed

✅ No issues found.