FN-5872: clear active-session registry on PR cleanup

Ensure PR-mode merge cleanup removes stale active-session registry state before deleting worktrees.

- unregister the task worktree from the active-session registry during merged-task artifact cleanup
- export the active-session registry from the engine entrypoint for lifecycle cleanup usage
- add CLI and engine regression coverage for registry cleanup and missing-entry best-effort behavior
- add a patch changeset for the published CLI package

Files changed:
 .changeset/fn-5872-registry-cleanup.md             |  5 ++++
 .../src/commands/__tests__/task-lifecycle.test.ts  | 29 +++++++++++++++++++++-
 packages/cli/src/commands/task-lifecycle.ts        |  8 +++++-
 .../pr-mode-worktree-invariants.test.ts            | 11 +++++---
 packages/engine/src/index.ts                       |  1 +
 5 files changed, 49 insertions(+), 5 deletions(-)

Fusion-Task-Id: FN-5872

Fusion-Task-Lineage: d4db24a3-7ae0-4d07-b029-7f30b2f65d2e

.changeset/fn-5872-registry-cleanup.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Clear stale active-session registry entries when PR-mode merge cleanup removes a task worktree.