FN-6168: expose workflow routing in triage

Add workflow discovery, selection, and investigation-task routing guidance to triage.

- document workflow routing guidance in standard and fast triage prompts
- add workflow list/select tools to triage sessions for task specification
- pass workflow_id and noCommitsExpected through fn_task_create for child tasks
- cover prompt/tool availability and child-task routing in triage tests
- add a changeset and workflow docs for triage workflow selection behavior

Files changed:
 .changeset/fn-6168-triage-workflow-routing.md |  5 ++
 docs/workflow-steps.md                        |  2 +
 packages/engine/src/__tests__/triage.test.ts  | 93 +++++++++++++++++++++++++++
 packages/engine/src/triage.ts                 | 27 ++++++++
 4 files changed, 127 insertions(+)

Fusion-Task-Id: FN-6168

Fusion-Task-Lineage: acfcebd5-bb9c-42ba-90a0-e9bcb90216b3

docs/workflow-steps.md

@@ -40,6 +40,8 @@ The default built-in catalog entry `builtin:coding` is backed by the canonical `
 
 `builtin:stepwise-coding` is a separate graph variant backed by `BUILTIN_STEPWISE_CODING_WORKFLOW_IR`; it keeps the same lifecycle columns/traits while modeling per-step parse/execute/review/rework as authored graph structure.
 
+During triage/planning sessions, agents can call `fn_workflow_list` to discover available built-in and custom workflows and read their descriptions before routing work. They can call `fn_workflow_select` to select a workflow for the task being specified, or pass `workflow_id` when creating child tasks with `fn_task_create`; decision-only or investigation tasks can also set `noCommitsExpected` / `**No commits expected:** true` when no code changes are expected.
+
 #### Runtime invariant criterion
 
 Workflow-driven coding runs must preserve observable task transitions and reliability invariants: file-scope guards including `FileScopeViolationError`, squash/merge contract, recovery expectations, `autoMerge:false` terminal-until-merged, and `moveTask(in-progress→todo)` hard-cancel semantics.