enhance(spec-system-map): add candidate capability boundaries handoff

[sungjunlee]

Summary

Strengthen spec-system-map as the bridge between the charter and capability contracts by adding a short Candidate Capability Boundaries section.

Proposed Shape

## Candidate Capability Boundaries
- `<slug>` — evidence: <flow/boundary/invariant>; owns <contract surface>; uncertainty: <what needs grill>

Acceptance Criteria

• skills/spec-system-map/templates/system-map.md includes Candidate Capability Boundaries before Where To Go Next.
• skills/spec-system-map/SKILL.md says create mode should use this section to hand off concrete candidates to spec-grill.
• The section is explicitly constrained to short boundary candidates, not module inventory.
• spec/system-map.md dogfood artifact is updated if appropriate.

Notes

Part of #176.

[coderabbitai]

🔗 Related PRs

#100 - docs: spec-system v0.1 design (layered spec architecture) [merged]
#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]
#160 - [codex] split spec charter and grill skills [merged]
#174 - feat(backlog-triage): add spec-aware Decision Review [closed]

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