refactor(FN-7039): drop the legacy workflow_steps table; remove all table readers

Final cutover — nothing reads workflow_steps at runtime, so migration 131 drops it.

- Removed the merger post-merge execution path entirely (runPostMergeWorkflowSteps,
  hasEnabledPostMergeWorkflowSteps, executePostMerge{Prompt,Script}Step, post-merge
  worktree helpers + call site). Graph owns post-merge.
- Executor recovery no longer reads getWorkflowStep().gateMode; gate-ness comes from the
  recorded WorkflowStepResult.status.
- Removed store CRUD (create/update/deleteWorkflowStep), materializeWorkflowSteps, and
  migrateLegacyWorkflowSteps; selectTaskWorkflow now seeds default-on optional-group node
  ids (consistent with create-time). KEPT the plugin step-template palette (getWorkflowStep
  plugin-only resolver / listWorkflowSteps plugin-only) — never touches the table.
  Removed the dashboard migrate-legacy-steps route + editor migration UI.
- SCHEMA_VERSION 130→131; migration 131 DROP TABLE IF EXISTS workflow_steps; SCHEMA_SQL
  table def removed; historical migrations 77/105/109/130 guarded with tableExists().

Proof nothing stranded: the graph executes IR nodes resolved from workflowId (never
stepIds/compiled rows) — materializeWorkflowSteps writes were vestigial. Full @fusion/core
suite (6290), reliability backstop (154), boot smoke, and a seed-at-130 drop test all pass
with the table gone.

Plan U7c.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/dashboard/app/api/legacy.ts

@@ -5464,22 +5464,8 @@ export function importWorkflow(
   });
 }
 
-/** Result of the lazy legacy-step migration (U2/R5). `migrated` is the number of
- *  newly converted user steps; `skipped` the count already migrated; when the
- *  defaultOn subset was non-empty a combined "Migrated steps" workflow id is set. */
-export interface MigrateLegacyStepsResult {
-  migrated: number;
-  skipped: number;
-  combinedWorkflowId?: string;
-}
-
-/** Run the lazy, idempotent migration of legacy user-authored workflow steps into
- *  fragments + a combined workflow (U2/R5). Safe to call repeatedly. */
-export function migrateLegacyWorkflowSteps(projectId?: string): Promise<MigrateLegacyStepsResult> {
-  return api<MigrateLegacyStepsResult>(withProjectId("/workflows/migrate-legacy-steps", projectId), {
-    method: "POST",
-  });
-}
+// FNXC:WorkflowStepCRUD 2026-06-26-14:00: U7c removed migrateLegacyWorkflowSteps and
+// MigrateLegacyStepsResult along with the legacy workflow_steps table and its route.
 
 /** Result of POST /api/workflows/design (U10/R11). The server validates the
  *  AI-produced IR (parseWorkflowIr), triages compilability (`interpreterOnly`),