spec-series: rename backlog-charter to spec-charter

[sungjunlee]

Problem

backlog-charter now owns the project spec axis, not just backlog integration. The name makes the CHARTER layer feel like a backlog-only feature and hides the broader spec-series direction.

Proposal

• Rename the skill directory and frontmatter from backlog-charter to spec-charter.
• Keep the skill focused on CHARTER.md create/amend.
• Update references from sibling skills and docs.
• Leave historical plan/spec files intact where they describe past implementation history.

Acceptance Criteria

• skills/spec-charter/SKILL.md exists and declares name: spec-charter.
• Active docs and sibling skills refer to spec-charter for CHARTER create/amend.
• No active instruction tells users to invoke backlog-charter for new work.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• feat(spec-system): rename backlog-charter surface to spec-charter #135

🔗 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]
#117 - docs(backlog-charter): note extract-signals clusters by code, not by capability [merged]
#156 - [codex] Fix backlog-charter skill metadata [merged]

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