#20544 ci: advisory familiar-intent review warning (no template field)

Summary

Split from #20395: the review-bot and workflow parts only. The PR template is unchanged.

• .github/review-bot-rules/familiar-intent.md and .coderabbit.yaml tell CodeRabbit and cubic to warn, and mention the marker's owner, when a hunk touches an @intent ... stability=familiar region and the description and diff don't show whether its familiar behavior is kept. Nothing is required of authors. An optional Intent-Delta: line (preserved, modified(...), retired(...) or new(...)) settles it. A missing line or none counts as no stated delta.
• intent-familiar-review.yml posts the same warning from scripts/ci/intent_familiar_warning.py. It is warning-only and never blocks.

Testing

• python3 tests/test_intent_familiar_warning.py (now also covers a missing line) and tests/test_cmux_next_checks_workflow.py pass locally. actionlint is clean on both workflows.

Changelog

none

Proof

CI only.

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