fix: Guard worktree creation against branch === baseBranch

Throws if branch and baseBranch are identical, preventing
git branch -f from force-resetting shared branches (like
the initiative branch) when accidentally passed as both.

apps/server/git/manager.ts

@@ -61,10 +61,18 @@ export class SimpleGitWorktreeManager implements WorktreeManager {
     const worktreePath = path.join(this.worktreesDir, id);
     log.info({ id, branch, baseBranch }, 'creating worktree');
 
+    // Safety: never force-reset a branch to its own base — this would nuke
+    // shared branches like the initiative branch if passed as both branch and baseBranch.
+    if (branch === baseBranch) {
+      throw new Error(`Worktree branch and baseBranch are the same (${branch}). Use a unique branch name.`);
+    }
+
     // Create worktree — reuse existing branch or create new one
     const branchExists = await this.branchExists(branch);
     if (branchExists) {
-      // Branch exists from a previous run — reset it to baseBranch and check it out
+      // Branch exists from a previous run — reset it to baseBranch and check it out.
+      // Only safe because branch !== baseBranch (checked above), so we're resetting
+      // an agent-scoped branch, not a shared branch like main or the initiative branch.
       await this.git.raw(['branch', '-f', branch, baseBranch]);
       await this.git.raw(['worktree', 'add', worktreePath, branch]);
     } else {