fix(skill): invalidate skill cache when SKILL.md files change on disk

[jwm4]

Issue for this PR

Closes #19050

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Skills are loaded once by ensure() in skill/index.ts and the result is cached as a resolved promise. Editing a SKILL.md file while opencode is running has no effect until restart because nothing clears that cache.

This fix adds an invalidate() function that resets the cached promise. It's triggered automatically when the existing FileWatcher detects a change to any SKILL.md file. The Command service also subscribes to a new Skill.Event.Invalidated bus event so slash commands pick up the fresh content.

Two source files changed, one new test file added:

• packages/opencode/src/skill/index.ts — cache invalidation + file watcher subscription
• packages/opencode/src/command/index.ts — subscribe to skill invalidation to rebuild commands
• packages/opencode/test/skill/skill-stale-cache.test.ts — 4 regression tests

How did you verify your code works?

• 4 new regression tests that modify SKILL.md on disk and call invalidate(), then verify Skill.list() returns updated content (edit, add, delete scenarios)
• All 11 existing skill tests pass
• All 9 skill tool tests pass
• Full test suite: 1468 pass, 0 fail, 17 skip
• Type checking passes (bun turbo typecheck)

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[jwm4]

Manual testing feedback

The cache invalidation plumbing works correctly — Skill.invalidate() clears the cache and the next Skill.all()/Skill.get() call re-reads from disk. The automated tests all pass.

However, there's a significant gap in the end-to-end flow: the fix only works when OPENCODE_EXPERIMENTAL_FILEWATCHER=true is set, which most users won't have enabled.

The PR subscribes to FileWatcher.Event.Updated to detect SKILL.md changes, but FileWatcher only watches Instance.directory when the experimental flag is on (see src/file/watcher.ts:123). Without it, only the .git directory is watched — so FileWatcher.Event.Updated never fires for SKILL.md changes, and the cache is never invalidated.

This means the original issue (#19050) remains unfixed for the default configuration.

Additional note

Even with the experimental flag enabled, the FileWatcher only watches Instance.directory (the CWD). In monorepo setups, skills loaded from parent .opencode/ directories (found via Filesystem.up) won't trigger invalidation when edited, since the watcher doesn't cover those parent directories.

Suggestions

1. Consider an approach that doesn't depend on OPENCODE_EXPERIMENTAL_FILEWATCHER — e.g., a dedicated watcher for skill directories, or polling skill files for changes.
2. If sticking with FileWatcher, the watcher scope should cover all directories returned by Config.directories() and the external skill dirs, not just Instance.directory.