FN-5818: fix shared-branch-group execution to use per-task working branches

Ensure shared branch-group flows consistently derive and acquire per-task working branches.

- update engine merge/executor/scheduler/self-healing paths to resolve task-scoped working branch names instead of shared branch-group names
- adjust worktree acquisition and already-merged detection logic to use the corrected branch resolution
- add regression coverage for shared-branch-group working-branch behavior, worktree acquisition, and worktree name derivation
- add a patch changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5818-shared-branch-working-branch.md |  5 ++
 .../shared-branch-group-working-branch.test.ts     | 53 ++++++++++++++++++++++
 .../src/__tests__/worktree-acquisition.test.ts     | 52 +++++++++++++++++++++
 .../engine/src/__tests__/worktree-names.test.ts    | 20 +++++++-
 packages/engine/src/already-merged-detector.ts     |  6 +--
 packages/engine/src/executor.ts                    | 10 ++--
 packages/engine/src/merger-ai.ts                   |  4 +-
 packages/engine/src/merger.ts                      | 16 +++----
 packages/engine/src/scheduler.ts                   |  6 +--
 packages/engine/src/self-healing.ts                | 12 ++---
 packages/engine/src/worktree-acquisition.ts        |  4 +-
 packages/engine/src/worktree-names.ts              |  9 +++-
 12 files changed, 166 insertions(+), 31 deletions(-)

Fusion-Task-Id: FN-5818

Fusion-Task-Lineage: c7eb1ec4-971a-49ad-9156-5c0349c629aa

.changeset/fn-5818-shared-branch-working-branch.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix shared branch-group execution to always derive per-task working branches (`fusion/<task-id>`) for checkout/worktree operations while keeping the branch-group branch as the merge target.