enhance(spec-grill): expand extract-signals into evidence-grouped collector

[sungjunlee]

Summary

Refactor extract-signals.js from directory/commit-scope candidate naming toward deterministic evidence grouping. The script should collect useful evidence for spec-grill, not decide accepted capabilities.

Evidence Sources To Add

• spec/system-map.md candidate boundary section
• README support/capability sections
• skills/*/SKILL.md names and descriptions
• skills/*/scripts/* command surfaces
• docs/reference guide files
• tests paired to scripts
• recent commit scopes

Acceptance Criteria

• extract-signals.js emits evidence grouped by candidate.
• Missing files degrade cleanly and appear as missing evidence, not failures.
• Duplicate evidence merges deterministically.
• The script does not auto-admit capabilities or write spec/capabilities.md.
• Tests cover system-map candidates, README sections, skill/script/test evidence, duplicate merging, and missing-file behavior.

Notes

Part of #176. Depends on the system-map candidate boundary section shape.

[coderabbitai]

🔗 Related PRs

#106 - docs(backlog-charter): create-mode interview + signals + seed-Decisions guidance (#93 #94 #96) [merged]
#117 - docs(backlog-charter): note extract-signals clusters by code, not by capability [merged]
#155 - Align spec-system harness signals [merged]
#160 - [codex] split spec charter and grill skills [merged]
#174 - feat(backlog-triage): add spec-aware Decision Review [closed]

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