fix(skills): keep unmodeled Copilot SKILL.md frontmatter through the round-trip

[dyoshikawa]

Summary

Two independent items from the Copilot follow-up issues.

(a) Residual-key skill round-trip (#2401)

CopilotSkillFrontmatterSchema is a z.looseObject, so a hand-authored SKILL.md field Rulesync does not model parsed fine — but toRulesyncSkill built the copilot: section by explicitly copying six known keys, so the field was dropped on import and then erased from the file on the next generate. Same fix class as #2580 applied to CopilotRuleFrontmatterSchema:

• toRulesyncSkill spreads all residual keys (everything except name/description, which have canonical homes) into the tool section; fromRulesyncSkill spreads the section back out, so the value survives regeneration rather than only living in .rulesync. The section is spread first, so the canonical name/description and the resolved invocation gates still own their keys.
• Applied to both copilot-skill.ts and copilotcli-skill.ts. The two targets write the same .github/skills/<name>/SKILL.md path and copilotcli is generated last, so fixing only copilot would have left the field dropped again in the common both-targets configuration.
• Documented in the copilot: and copilotcli: skills sections of docs/reference/file-formats.md, including the caveat that a residual key rides one section only, exactly like the modeled fields.

Note: because the section is now spread before name/description, generated SKILL.md files put the section keys first. This is a key-order change only, no semantic difference, and it matches what copilot-rule.ts already does.

(b) Stale comment (#2402)

copilotcli-hooks.ts said the global hooks location was "a rulesync convention pending official documentation". Upstream documents it: "User-level hook files — *.json files in the user-level hooks directory. By default this is ~/.copilot/hooks/ on macOS and Linux, or %USERPROFILE%\\.copilot\\hooks\\ on Windows" (github/docs content/copilot/reference/hooks-reference.md). The comment now says the directory is documented and only the filename is Rulesync's choice, and notes that COPILOT_HOME relocates it upstream while Rulesync does not read that variable yet.

Testing

• Round-trip test in each of copilot-skill.test.ts / copilotcli-skill.test.ts: a field beyond the schema survives import into the tool section and is written back to the generated SKILL.md.
• A precedence test in each: a section carrying name/description does not shadow the canonical values, while its other keys are still kept.
• pnpm cicheck green; vitest run --config vitest.e2e.config.ts src/e2e/e2e-skills.spec.ts green (130 tests).

Part of #2401
Part of #2402