FN-5782: wire branch groups into merge routing

Connect branch_group metadata through planning and merge execution for grouped integration branches.

- propagate `branchGroup` and `branchGroupName` through core store types, merge metadata, and CLI task lifecycle APIs
- add merge coordination that computes branch-group plans and routes grouped tasks via `group-merge-coordinator`
- extend finalize-plan, merger, and reliability tests to cover grouped merge routing and integration behavior
- document the architecture update and add a patch changeset for `@runfusion/fusion`

Files changed:
 .changeset/fn-5782-branch-group-merge.md           |   7 +
 docs/architecture.md                               |   2 +
 packages/cli/src/commands/__tests__/task-lifecycle.test.ts  |  64 ++++++++-
 packages/cli/src/commands/task-lifecycle.ts        |   4 +
 packages/core/src/__tests__/branch-group-store.test.ts  |  20 +++
 packages/core/src/__tests__/task-merge.test.ts     |  64 +++++++++
 packages/core/src/store.ts                         |  22 ++-
 packages/core/src/task-merge.ts                    |  26 +++-
 packages/core/src/types.ts                         |   2 +-
 packages/engine/src/__tests__/experiment-finalize-plan.test.ts |  33 ++++-
 packages/engine/src/__tests__/group-merge-coordinator.test.ts  |  62 +++++++++
 packages/engine/src/__tests__/reliability-interactions/branch-group-merge-routing.test.ts | 153 +++++++++++++++++++++
 packages/engine/src/experiment/finalize-plan.ts    |  62 +++++++--
 packages/engine/src/group-merge-coordinator.ts     |  51 +++++++
 packages/engine/src/index.ts                       |   4 +
 packages/engine/src/merger.ts                      |  47 ++++++-
 16 files changed, 601 insertions(+), 22 deletions(-)

Fusion-Task-Id: FN-5782
Fusion-Task-Lineage: 0a70cfaa-2379-4955-b295-64de1f3093c8

docs/architecture.md

@@ -1575,6 +1575,7 @@ The GitHub tracking state listener now attaches to every registered project stor
 ### Merge strategies
 - Setting type: `MergeStrategy = "direct" | "pull-request"` (`types.ts`)
 - `aiMergeTask()` in `merger.ts` performs merge flow
+- FN-5782 wires branch-group routing into merge target resolution: tasks with `branchContext.assignmentMode === "shared"` and a resolvable `branch_groups` row merge onto `branch_groups.branchName` (`mergeTarget.source = "branch-group-integration"`) instead of the project default branch; ungrouped and `per-task-derived` tasks keep the existing direct-to-default path unchanged. Merge emits `merge:branch-group-routed` audit telemetry for routed members. Promotion of the group branch back to the project default branch remains a downstream task.
 - FN-5279 adds `mergeIntegrationWorktree` for auto-merge only. Default `reuse-task-worktree` hands merger ownership from executor to the merger inside the task worktree after five gates (clean tree, expected branch, no live executor session, canonical branch/worktree binding, lease handoff). Refusals emit `merge:reuse-handoff-refused`, leave the task in `in-review`, and do **not** silently fall back to project-root merge mode. `cwd-integration-branch` is the explicit opt-in project-root path; `cwd-main` is a deprecated alias normalized to `cwd-integration-branch`. Integration-branch defaults across merger and self-healing flows are resolved dynamically via `resolveIntegrationBranch(rootDir, settings)` (`integrationBranch` → `baseBranch` → `origin/HEAD` → `main`). When `worktrunk.enabled=true`, worktrunk-managed merge/worktree behavior still wins and the handoff path emits a defer event instead of taking over. FN-5363 tightens this path: `acquireMergeQueueLease({ targetTaskId })` is strict (no queue-head fallback), merge queue rows are enqueue/lease-gated to `in-review` tasks, and stale non-review rows are auto-cleaned (including on `in-review` column exit when leases are absent or expired). FN-5353 extends the same contract: merger self-enqueues the target before strict target leasing, null target leases are surfaced as `merge:reuse-handoff-refused` with `reason: "target-not-queued"`, `acquireReuseHandoff` hard-refuses `reason: "worktree-equals-project-root"`, and `resolveMergeIntegrationRoot` returns a missing-worktree sentinel (`rootDir: ""`) so reacquire executes before any reuse gate can misroute against project root. FN-5351 adds a production verification trail for integration-branch invariants: `merge:integration-worktree-state`, `merge:cwd-integration-fallback-refused`, and `merge:integration-ref-advance`.
 - `merger.ts` also exposes a test-only `__test__` helper object for internal merger unit/integration coverage (for example autostash orphan cleanup behavior)
 - Supports workflow-step execution after merge (post-merge phase)
@@ -1771,6 +1772,7 @@ Reliability-layer changes are in scope. Interaction regression backstops live in
 - FN-5219 backstop: `packages/engine/src/__tests__/reliability-interactions/in-progress-limbo-recovery.test.ts` covers `recoverInProgressLimbo` composition with `recoverOrphanedExecutions` (no double-recovery), `reconcile-task-worktree-metadata` (live rebindable worktree wins), `recoverMissingWorktreeReviewFailures` (in-review vs in-progress disjoint), and executor task-id claim skip, plus an explicit FN-5149 reproduction case.
 - FN-5704 backstop: `packages/engine/src/__tests__/reliability-interactions/reclaim-self-owned-resume-limbo-escalation.test.ts` covers bounded no-progress reclaim/resume detection, preserve-work escalation to `todo`, `task:resume-limbo-escalated` audit metadata, progress-signal reset behavior, and user-paused/autoMerge-off non-escalation guards.
 - FN-5715 backstop: `packages/engine/src/__tests__/reliability-interactions/mission-validation-trigger-gap.test.ts` locks the mission-validation trigger invariant so done mission-linked tasks still start validation when the mission loop was stopped, startup recovery replays done implementing features with unpassed assertions, and recovery remains idempotent for already-passed features.
+- FN-5782 backstop: `packages/engine/src/__tests__/reliability-interactions/branch-group-merge-routing.test.ts` guards branch-group merge routing so `shared` members land on `branch_groups.branchName`, grouped multi-member merges converge on the same integration branch, ungrouped/`per-task-derived` tasks stay on direct default-branch merge flow, and routed merges emit `merge:branch-group-routed` audit metadata.
 - FN-5738 backstop: `packages/engine/src/__tests__/reliability-interactions/mission-validation-trigger-gap.test.ts` extends coverage so zero-assertion auto-pass deterministically advances `loopState` to `passed`, sets `lastValidatorStatus="passed"`, emits `validation_auto_passed_no_assertions`, and does not re-fire on repeated recovery passes.
 - FN-5741 backstop: `packages/engine/src/__tests__/reliability-interactions/merge-request-shadow-handoff.test.ts` guards Phase-1 merge-request contract shadow writes: flag OFF is a no-op, flag ON writes marker/record strictly after legacy handoff, and `autoMerge:false` remains `manual-required` without shadow running transitions.
 - FN-5742 backstop: `packages/engine/src/__tests__/reliability-interactions/dual-observe-merge-seam.test.ts` guards Phase-2 dual-observe invariants: legacy dependency satisfaction remains authoritative while parity diffs emit, and shadow dequeue selection never advances `manual-required` rows.