docs(backlog-triage): resolve spec-charter naming and charter path fallback

[sungjunlee]

Context

The spec axis has moved from backlog-charter / root CHARTER.md to spec-charter / spec/charter.md, with root CHARTER.md as a legacy fallback only.

Part of #167.

Acceptance Criteria

• backlog-triage docs refer to spec-charter as the owner of charter create/amend/reassess.
• Alignment Check reads spec/charter.md first and falls back to legacy root CHARTER.md only when needed.
• Missing charter remains a graceful no-op.
• References to backlog-charter remain only in historical docs or are clearly marked legacy.

[coderabbitai]

🔗 Related PRs

#92 - promote: CHARTER reference axis into main [merged]
#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]

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[sungjunlee]

Implemented in skills/backlog-triage/SKILL.md and references/decision-review.md. backlog-triage now names spec-charter as charter owner, reads spec/charter.md before legacy root CHARTER.md, and treats missing charter as a graceful no-op. Verified with node --test skills//scripts/.test.js and bash skills/dev-backlog/scripts/smoke-test.sh.