feat(label): add hook auto-labeling issues and pull requests

[Osheun]

What

New label hook, as requested in #36. Just like assign, it is rule-based:

• Keyword rules — case-insensitive, whole-word title matching, applied to both issues and PRs
• Path rules — glob matching (minimatch, same as assign) against a PR's changed files, PRs only

Both rule sets ship with sensible defaults that kick in when the hook is enabled without further config:

• keywords: bug (fix, bug, crash, error, broken), enhancement (feat, feature, add, improve, support), documentation (docs, documentation, readme, typo)
• paths: documentation (**/*.md, docs/**), tests (tests/**, **/*.test.*, ...), dependencies (package.json, **/*.lock, ...)

Defining your own keywords/paths replaces the defaults. Matched labels are deduplicated and applied in a single addLabels call; no API call is made when nothing matches.

hooks:
  label:
    enabled: false # default

Changes

• src/app/hooks/label/ — issueOpen.ts (issues.opened), prOpen.ts (pull_request opened/reopened/synchronize), pure matchers.ts, schema.ts
• schema registered in src/schemas/hooks.ts
• documented in HOOKS.md with a config example
• tests in tests/app/hooks/label.test.ts covering matchers, defaults, custom rules, dedup and disabled state

No new permissions/events needed (issues: write and the issues/pull_request events are already in app.yml).

Closes #36

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