FN-5766: add flagged-off workflow graph executor scaffold

Add the Phase 2 workflow-graph interpreter scaffold and builtin coding IR wiring while keeping execution behavior unchanged by default.

- add BUILTIN_CODING_WORKFLOW_IR and builder in @fusion/core with coverage tests
- export new coding workflow IR APIs from core index
- add WorkflowGraphExecutor scaffold in @fusion/engine plus parity-focused test coverage
- document the flagged-off interpreter scaffold, parity gate, and v1 IR gap reconciliation in workflow docs
- include a changeset for published @runfusion/fusion

Files changed:
 .../fn-5766-workflow-graph-executor-scaffold.md    | 11 +++
 docs/workflow-steps.md                             | 27 +++++++
 .../__tests__/builtin-coding-workflow-ir.test.ts   | 34 +++++++++
 packages/core/src/builtin-coding-workflow-ir.ts    | 60 ++++++++++++++++
 packages/core/src/index.ts                         |  4 ++
 .../workflow-graph-executor-parity.test.ts         | 32 +++++++++
 packages/engine/src/index.ts                       |  7 ++
 packages/engine/src/workflow-graph-executor.ts     | 82 ++++++++++++++++++++++
 8 files changed, 257 insertions(+)

Fusion-Task-Id: FN-5766
Fusion-Task-Lineage: 4bbef713-a415-4d42-a20b-9ae4f3235419

docs/workflow-steps.md

@@ -28,6 +28,33 @@ Out of scope for v1:
 - Execution history/runtime traces
 - Migration tooling for future schema versions (future versions should use explicit `schemaVersion` migrations)
 
+### Workflow Graph Executor (interpreter scaffold)
+
+FN-5766 adds a **flagged-off** interpreter scaffold in `@fusion/engine` (`WorkflowGraphExecutor`) plus a built-in coding lifecycle IR in `@fusion/core` (`BUILTIN_CODING_WORKFLOW_IR`).
+
+- Feature flag key: `experimentalFeatures.workflowGraphExecutor`
+- Default: **OFF**
+- OFF behavior is strict no-op (no task mutations, no session/git side effects), so the legacy imperative pipeline remains authoritative.
+
+Built-in coding IR currently encodes the legacy lifecycle path as graph stages:
+
+- `triage` → `execute` → `review` → `merge` → `end`
+
+#### Interpreter-parity gating criterion
+
+Interpreter authority is gated on parity: interpreter-driven coding runs must match legacy behavior for observable task transitions and reliability invariants (file-scope guards including `FileScopeViolationError`, squash/merge contract, self-healing expectations, `autoMerge:false` terminal-until-merged, and `moveTask(in-progress→todo)` hard-cancel semantics).
+
+#### IR-gap reconciliation (v1)
+
+The workflow redesign brief references `agent-call` nodes and typed edges (`success|failure|conditional|fan-out-join`), but shipped v1 IR only supports node kinds `start|prompt|script|gate|end` plus optional string edge `condition`.
+
+Current reconciliation in v1:
+
+- `agent-call` semantics are represented using existing `prompt` nodes with `config` fields (for example stage/role metadata).
+- Typed-edge semantics are represented using `condition` token conventions.
+
+Potential first-class schema support for `agent-call`/typed edges is deferred to a follow-up IR extension task (v1.1 candidate), not part of v1 contract changes.
+
 ## What They Are
 
 A workflow step is a reusable check (AI prompt or script) that can be enabled on tasks.