fix: defer Herdr away injection without atomic admission

[pranaypratyush]

Summary

• Defer Herdr away-mode delivery whenever the backend cannot atomically admit a confirmed-empty composer and submit the digest.
• Preserve the escalation buffer and human draft; never use a generic compare/send fallback for Herdr.
• Cover the race deterministically and in a named non-default Herdr lab.

Validation

• tests/fm-daemon.test.sh
• tests/fm-backend-herdr.test.sh
• HERDR_LAB_HELPER=/home/pranay/wd/firstmate/bin/fm-herdr-lab.sh tests/fm-afk-inject-herdr-e2e.test.sh
• bin/fm-lint.sh
• bin/fm-doc-audience-check.sh
• bin/fm-test-run.sh --all and the changed selection were run. Their two concrete red results reproduce unchanged on clean origin/main: the Herdr presentation fixture-worktree reuse check and the watcher warning-text assertion.
• The Pi/Herdr opt-in smoke was skipped because pi is unavailable on this machine.

No merge requested.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-31T11:13:03.083824Z	9f0129d	PR opened

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