fix: Worktree get() matches wrong agent due to ambiguous endsWith lookup

The `get(id)` method on SimpleGitWorktreeManager used `path.endsWith(id)`
to find worktrees. Since all agents working on the same project create
worktrees with the same project name suffix (e.g., "codewalk-district"),
cleanup for one agent could match and delete another agent's worktree.

Fix: match on `basename(worktreesDir)/id` so each manager's lookups are
scoped to its own worktree base directory.

apps/server/git/manager.test.ts

@@ -453,6 +453,58 @@ describe('SimpleGitWorktreeManager', () => {
     });
   });
 
+  // ==========================================================================
+  // Cross-Agent Isolation
+  // ==========================================================================
+
+  describe('cross-agent isolation', () => {
+    it('get() only matches worktrees in its own worktreesDir', async () => {
+      // Simulate two agents with separate worktree base dirs but same repo
+      const agentADir = path.join(repoPath, 'workdirs', 'agent-a');
+      const agentBDir = path.join(repoPath, 'workdirs', 'agent-b');
+      await mkdir(agentADir, { recursive: true });
+      await mkdir(agentBDir, { recursive: true });
+
+      const managerA = new SimpleGitWorktreeManager(repoPath, undefined, agentADir);
+      const managerB = new SimpleGitWorktreeManager(repoPath, undefined, agentBDir);
+
+      // Both create worktrees with the same id (project name)
+      await managerA.create('my-project', 'agent/agent-a');
+      await managerB.create('my-project', 'agent/agent-b');
+
+      // Each manager should only see its own worktree
+      const wtA = await managerA.get('my-project');
+      const wtB = await managerB.get('my-project');
+
+      expect(wtA).not.toBeNull();
+      expect(wtB).not.toBeNull();
+      expect(wtA!.path).toContain('agent-a');
+      expect(wtB!.path).toContain('agent-b');
+      expect(wtA!.path).not.toBe(wtB!.path);
+    });
+
+    it('remove() only removes worktrees in its own worktreesDir', async () => {
+      const agentADir = path.join(repoPath, 'workdirs', 'agent-a');
+      const agentBDir = path.join(repoPath, 'workdirs', 'agent-b');
+      await mkdir(agentADir, { recursive: true });
+      await mkdir(agentBDir, { recursive: true });
+
+      const managerA = new SimpleGitWorktreeManager(repoPath, undefined, agentADir);
+      const managerB = new SimpleGitWorktreeManager(repoPath, undefined, agentBDir);
+
+      await managerA.create('my-project', 'agent/agent-a');
+      await managerB.create('my-project', 'agent/agent-b');
+
+      // Remove agent A's worktree
+      await managerA.remove('my-project');
+
+      // Agent B's worktree should still exist
+      const wtB = await managerB.get('my-project');
+      expect(wtB).not.toBeNull();
+      expect(wtB!.path).toContain('agent-b');
+    });
+  });
+
   // ==========================================================================
   // Edge Cases
   // ==========================================================================