refactor(core): unify compatibility skill loading

[opencode-agent]

Summary

• load Claude and Agents compatibility directories through ConfigSkillPlugin as ordinary directory sources
• remove the duplicate compatibility watcher, scanner, parser, and reload lifecycle
• emit config updates when compatibility roots appear so late-created skill directories use the shared watcher path

Testing

• bun test test/config/config.test.ts test/config/skill.test.ts (50 passed)
• bun typecheck in packages/core
• scoped oxlint (passed)
• bun run check lint passed; the repo-wide typecheck hit the host OS thread limit in unrelated @opencode/console-support

Requested by: @thdxr (dax via Slack)