feat(FN-000): extract workflow merge node capability

Fusion-Task-Id: FN-000

docs/plans/workflow-owned-merge-stack/s06-git-merge-capabilities.md

@@ -0,0 +1,46 @@
+---
+title: "S06: git and merge capability extraction"
+type: refactor
+status: draft-stack-handoff
+date: 2026-06-09
+slice: S06
+milestone: "Runtime"
+origin: docs/plans/2026-06-09-003-refactor-workflow-owned-merge-full-migration-slices-plan.md
+stack_base: feature/workflow-owned-merge-s05-runtime-work-item-driver
+---
+
+# S06: git and merge capability extraction
+
+## Stack Role
+
+This draft PR reserves the S06 review slot in the workflow-owned merge,
+retry, scheduling, and recovery migration stack. It is intentionally a handoff
+artifact, not the completed implementation for this slice.
+
+## Milestone
+
+Runtime
+
+## Depends On
+
+S4 built-in IR regions and S5 runtime work-item driver.
+
+## Goal
+
+Put checkout preparation, branch integration, merge attempt, squash, finalize, and conflict classification behind workflow node capability modules.
+
+## Expected File Scope
+
+packages/engine/src/merger*.ts; packages/engine/src/workflow-merge-nodes.ts; merge capability tests.
+
+## Expected Tests
+
+Checkout preparation, file-scope failure, already-on-main finalize, transient retry, permanent conflict routing, and guard-service coverage.
+
+## Exit Gate
+
+A merge attempt can be driven by a workflow node capability with the same guard behavior as merger.ts.
+
+## Full Plan
+
+See `docs/plans/2026-06-09-003-refactor-workflow-owned-merge-full-migration-slices-plan.md`.