feat: migrate to OpenCode SDK worktrees with per-project config

Add SDK-based worktree management that lists and starts SDK worktrees
Migrate per-project setup to ~/.config/openchamber/<projectId>.json
Deprecate .openchamber legacy paths and adapt UI to new config

packages/ui/src/lib/gitApiHttp.ts

@@ -325,6 +325,7 @@ export async function removeGitWorktree(directory: string, payload: GitRemoveWor
 }
 
 export async function ensureOpenChamberIgnored(directory: string): Promise<void> {
+  // LEGACY_WORKTREES: only needed for <project>/.openchamber era. Safe to remove after legacy support dropped.
   const response = await fetch(buildUrl(`${API_BASE}/ignore-openchamber`, directory), {
     method: 'POST',
   });