feat: sync skills directory by default

[iHildy]

Summary

• sync ~/.config/opencode/skills/ by default alongside other config directories
• filter skills from extraConfigPaths so there is a single canonical sync location
• document the default behavior and add focused path-plan coverage

Testing

• bun run check
• bun run test

Fixes #40

[gemini-code-assist]

Code Review

This pull request adds the skills directory to the default configuration directories for synchronization, updating the documentation, tests, and path resolution logic. The changes ensure that the skills folder is synced by default and filtered out if manually specified in extraConfigPaths. Feedback was provided to generalize the path filtering logic in src/sync/paths.ts to exclude any default sync item from the extra configuration paths, which would improve maintainability by avoiding hardcoded values.

[jules-relay]

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