feat: add support for syncing skills directory

Add `includeSkills` option to sync configuration, which defaults to true.
This allows the `~/.config/opencode/skills/` directory to be synced
alongside other configuration files and directories.

Updated:
- `src/sync/config.ts`: Added `includeSkills` to interfaces and normalization logic.
- `src/sync/paths.ts`: Added `skills` directory to sync plan items.
- `src/index.ts`: Exposed `includeSkills` in `opencode_sync` tool arguments.
- `src/sync/service.ts`: Handled `includeSkills` in init and status commands.
- `README.md`: Documented the new option and synced path.
- `src/sync/config.test.ts` & `src/sync/paths.test.ts`: Added tests for the new feature.

Co-authored-by: iHildy <25069719+iHildy@users.noreply.github.com>

Author: google-labs-jules[bot]

src/index.ts

@@ -152,10 +152,7 @@ export const opencodeConfigSync: Plugin = async (ctx) => {
         .boolean()
         .optional()
         .describe('Sync model favorites (state/model.json)'),
-      includeSkills: tool.schema
-        .boolean()
-        .optional()
-        .describe('Sync skills directory'),
+      includeSkills: tool.schema.boolean().optional().describe('Sync skills directory'),
       create: tool.schema.boolean().optional().describe('Create repo if missing'),
       private: tool.schema.boolean().optional().describe('Create repo as private'),
       extraSecretPaths: tool.schema.array(tool.schema.string()).optional(),