Follow-up: 4 P2/Medium review findings on #466 (pre-push scope edges + hook chunk ID)

[EtanHey]

Merged #466 on green per merge policy (no critical/high); these P2/Medium findings carry over:

1. P2: include nested pytest files in changed-only scope (BRAINLAYER_PREPUSH_SCOPE)
2. P2: fall back when any changed source is unmapped
3. Medium (Bugbot): changed-only with empty changed_files skips the suite entirely — should fall back to scoped default
4. P2: return the canonical hook chunk ID for embedding when a queued duplicate merges (touches hooks/indexer.py — the realtime stamping path; empirical stamping verified post-deploy by LEAD re-probe, this is the ID-canonicalization nuance only)

Source threads: #466 (inline comments). 🤖 brainlayerClaude-LEAD-v2