spec-series: split capability contract authoring into spec-grill

[sungjunlee]

Problem

The old backlog-charter grill mode is the right workflow for spec/capabilities.md, but it is hidden behind a CHARTER-named skill. On existing repos, users naturally need a second layer after CHARTER.md; without a discoverable skill they stop too early.

Proposal

• Create skills/spec-grill/SKILL.md for spec/capabilities.md create/refine.
• Move the capabilities template, extract-signals helper, tests, and grill heuristics under spec-grill.
• Make brownfield/onboarding language explicit: after spec-charter create, existing repos should be guided to spec-grill.

Acceptance Criteria

• skills/spec-grill/SKILL.md exists and declares name: spec-grill.
• spec-grill frontmatter discovers capability specs, component contracts, repo boundaries, and grill phrasing.
• extract-signals output points to spec-grill, not backlog-charter grill.
• Existing tests for signal extraction still pass.

[coderabbitai]

🔗 Related PRs

#106 - docs(backlog-charter): create-mode interview + signals + seed-Decisions guidance (#93 #94 #96) [merged]
#110 - docs: dogfood spec/capabilities.md for dev-backlog itself (#105) [merged]
#117 - docs(backlog-charter): note extract-signals clusters by code, not by capability [merged]
#155 - Align spec-system harness signals [merged]
#156 - [codex] Fix backlog-charter skill metadata [merged]

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