docs(file-formats): note comma-separated form for claudecode skill paths

[Chen17-sq]

Summary

Addresses item 3 from #1618 (review findings for PR #1604, the Claude Code skills paths field).

ClaudecodeSkillFrontmatterSchema declares:

// src/features/skills/claudecode-skill.ts:27
paths: z.optional(z.union([z.string(), z.array(z.string())])),

so paths accepts a YAML list or a comma-separated string. The docs/reference/file-formats.md SKILL.md example only showed the list form, leaving the string form undocumented.

This PR adds a one-sentence callout immediately after the SKILL.md example that names the comma-separated form and shows the equivalent value:

claudecode.paths accepts either the YAML list shown above or a comma-separated string (e.g. paths: "src/**/*.ts,test/**/*.ts"); the two forms are equivalent.

No code change. No schema change. skills/rulesync/file-formats.md is auto-regenerated from docs/ by scripts/sync-skill-docs.ts (wired through lint-staged on every docs/**/*.md commit) and is included in this PR for that reason.

Test plan

• pnpm cicheck (fmt + oxlint + eslint + tsgo + 5550 vitest + sync-skill-docs check + cspell + secretlint) passes locally on Node 22.
• Diff is a single inserted sentence in docs/reference/file-formats.md plus the mirrored regeneration in skills/rulesync/file-formats.md.
• The note is placed outside the existing SKILL.md example fence, so it renders as parsed Markdown rather than as code.

Refs #1618 (item 3 of 6 — the other follow-ups in that issue are out of scope here; item 4 is being addressed separately in #1648).

[github-actions]

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[Chen17-sq]

Closing this proactively — the github-actions bot flagged that I'm well over the 2-PR limit for external contributors documented in CONTRIBUTING.md. I opened these nine PRs in one batch without spotting that rule first, which was my mistake — sorry for the noise.

The patch in this PR is ready as-is (pnpm cicheck green, lint-staged green, pre-commit pnpm dev generate green) and stays small enough for a quick read; reopening is one click.

Keeping #1643 (refactor(utils): consolidate isPlainObject, addresses #1638) and #1644 (refactor(copilot-rule): reuse toPosixPath, addresses #1603) as my current two. I'll reopen this one — addressing #1618 item 3 — once a slot frees up.