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/api/types.ts

@@ -367,7 +367,6 @@ export interface FilesAPI {
 }
 
 export interface WorktreeDefaults {
-  branchPrefix?: string;        // e.g. "feature", "bugfix" (no trailing slash)
   baseBranch?: string;          // e.g. "main", "develop", or "HEAD"
   autoCreateWorktree?: boolean; // future: skip dialog, create worktree automatically
 }