feat(core): refresh global, external, and HTTP skill sources without restart

[neerajvipparla]

Issue for this PR

Closes #37426

Type of change

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

What does this PR do?

Extends V2 skill-content refresh beyond skill files inside watched git project Locations.

The issue: project .opencode/skills content already invalidates through the Location filesystem watcher, but global compatibility roots, explicitly configured external directories, non-git Locations, and cached HTTP catalogs had no continuous invalidation path. Replaying an unchanged source registration also did not clear its cached content.

Changes made:

• Added a config-external-skill plugin that registers ~/.claude/skills, ~/.agents/skills, and project-relative .claude/.agents skill directories as SkillV2.DirectorySource sources.
• SkillV2 now watches each directory source by its real path and invalidates cached skills when the watcher reports changes.
• SkillV2 manages its own per-source watch scopes, so skill refresh works whether or not the Location filesystem watcher is enabled.
• SkillV2 clears cached URL sources on a 60-minute schedule so remote catalogs can be observed while OpenCode remains running.
• A reconcile pass clears cache entries for removed sources and tears down watches for directories that are no longer registered, so skill removal is reflected on the next safe boundary.
• Added opencode debug skill-v2 and a --watch flag for live skill listing.
• Added/updated tests covering project, global, external, non-git, and HTTP source behavior in packages/core and packages/opencode.

How did you verify your code works?

• Ran the affected skill tests in packages/core: bun test test/skill.test.ts test/config/external-skill.test.ts test/plugin/skill.test.ts (9 pass).
• Ran the affected skill tests in packages/opencode: bun test test/skill/skill.test.ts (pass).
• Ran bun typecheck in packages/core and packages/opencode (pass).
• Ran oxlint on the changed files (0 errors).
• The CLI help-snapshot test failure is pre-existing on dev (process exits -1 in the test harness but not when run directly), so it is not introduced by this PR.

Screenshots / recordings

N/A - this is not a UI change.

Checklist

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

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