docs: add session-compact to built-in skills

[codeaholicguy]

Verified findings

• Doc claim (7-skills.md): built-in table lists 23 skills. Reality: skills/built-in.json has 24, including session-compact.
• Doc claim: built-ins described as a static curated set. Reality: served from a remote manifest since 0.61 with an embedded offline fallback — new built-ins can ship without a CLI release.
• Adjusted finding: the "repository also contains" note omits changelog and refactor, which the registry also ships.

What changed

• Added session-compact row to the built-in skills table
• One-line note on the remote manifest + offline fallback
• Expanded the repo-skills note with changelog and refactor

Validation

• npm run build in web/: pass
• npm run lint at root: pass

Generated with Devin

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technical-writer review pass: ran; no issues flagged.