fix(dashboard): prevent foreign-branch diffs after worktree pool reuse

When the worktree-recycle pool reassigned a path to a new task, the old
task's diff endpoints kept reading the new task's branch state — surfacing
unrelated commits as the original task's "files changed" list.

- Clear task.worktree/branch in the merger after the worktree is released
  to the pool or removed, so the path no longer points anywhere.
- Validate the worktree's current branch matches task.branch in the three
  worktree-backed diff endpoints; on mismatch return empty rather than
  diffing against a foreign branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/src/routes/resolve-diff-base.ts

@@ -47,6 +47,12 @@ export interface ResolveDiffBaseTaskInput {
  * 2. **Task-scoped baseCommitSha** — If merge-base is unavailable or equals
  *    `headRef`, use `baseCommitSha` when still an ancestor of `headRef`.
  * 3. **headRef~1** — Last-resort fallback.
+ *
+ * Note: callers must validate the worktree still belongs to the task (e.g.
+ * compare `git rev-parse --abbrev-ref HEAD` to `task.branch`) before invoking
+ * this. After worktree-pool reassignment the same path may host a foreign
+ * branch, in which case `baseCommitSha..HEAD` would surface other tasks'
+ * commits and this function has no way to detect that.
  */
 export async function resolveDiffBase(
   task: ResolveDiffBaseTaskInput,