docs(spec-charter): recommend concrete next steps without argument memorization

[sungjunlee]

Summary

Make spec-charter completion guidance concrete without asking users to memorize downstream command arguments. The charter should recommend the next plain-language action and candidate boundaries when evidence supports them.

Acceptance Criteria

• Create/amend/reassess completion recommends spec-system-map first when spec/system-map.md is missing on a brownfield repo.
• When system-map evidence exists and capabilities are missing or thin, completion recommends asking spec-grill to review candidate capability boundaries.
• Recommendations include 2-5 candidate names only when backed by repo evidence.
• Wording avoids making map, fill, or audit required user knowledge.

Notes

Part of #176. Should align with #177 and the system-map handoff work.

[coderabbitai]

🔗 Related PRs

#160 - [codex] split spec charter and grill skills [merged]

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