chore(engine): remove workflow-step mock routing + stale FN-5482 docs

Drops the "workflow-step" MockSessionPurpose enum value and the
workflowStepId / workflowStepTemplateId plumbing through
agent-runtime, agent-session-helpers, mock-provider, executor, and
merger. The seeded-workflow-prompts script loses its FN-5205
rationale comment + test (no longer applicable now that workflow
steps run through the regular session purposes).

Also strips the stale FN-5482 architecture-invariant bullet from
AGENTS.md and the corresponding audit-event line from
docs/architecture.md (the self-healing reclaim invariant they
described no longer holds).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture.md

@@ -1062,7 +1062,6 @@ The run-audit system records every mutation performed by the engine across four
 - **Database / `task:auto-recover-misrouted-foreign-commit`** — emitted per dropped misrouted commit during FN-4948 contamination recovery. `target` is the recovering task; metadata carries `{ droppedSha, foreignTaskId, paths }`.
 - **Database / `task:orphan-detected-no-action`** — emitted by `recoverOrphanedExecutions` (FN-5337) when row metadata looks orphaned after grace windows; annotation-only event with no lifecycle mutation (`in-progress` task stays put).
 - **Database / `task:*-no-action` backward-move family (FN-5335)** — backward self-healing sweeps now emit annotation-only events when triple proof fails instead of mutating lifecycle state. New mutation types: `task:reclaim-pr-conflict-no-action`, `task:reclaim-self-owned-branch-conflict-no-action`, `task:auto-rebound-scope-decay-no-action`, `task:finalize-no-op-review-no-action`, `task:stale-incomplete-review-no-action`, `task:ghost-review-no-action`, `task:stuck-merge-deadlock-no-action`, `task:no-progress-no-task-done-no-action`, `task:missing-worktree-review-no-action`, `task:partial-progress-no-task-done-no-action`. See `docs/self-healing-backward-move-audit.md` for per-stage disposition.
-- **Database / `task:auto-recover-reclaim-self-owned-routed-to-review` (FN-5482)** — emitted when self-healing finds preserved commits on a self-owned branch while the row is stranded in `todo` and routes it to `in-review` for explicit handoff (preserving `worktree`/`branch`/`baseCommitSha`). Metadata shape: `{ taskId, branch, worktreePath, preservedCommitCount, tipSha, sourceColumn, priorAssignedAgentId, reason }`, where `reason ∈ { "preserved-commits-with-stale-todo-metadata", "stranded-todo-reconcile" }`.
 - **Filesystem** — file:write, prompt:write, attachment:create, etc.
 - **Sandbox** — backend lifecycle events from `SandboxBackend` wiring in executor/merger/routine-runner (`sandbox:prepare`, `sandbox:run`, `sandbox:failure`, `sandbox:fallback`) introduced after FN-4636.
 

docs/workflow-steps.md

@@ -79,8 +79,6 @@ All seven built-in templates emit the structured `{"verdict":"APPROVE|APPROVE_WI
 
 The **Browser Verification** template uses browser automation style checks and is designed for UI validation flows.
 
-**Test mode behavior:** when Fusion test mode is active (`testMode: true` or `defaultProvider === "mock"`), Browser Verification routes to the FN-5203 mock provider and returns the canonical `{"verdict":"APPROVE","notes":""}` output without spawning a real model or `agent-browser`. Tests can force gate failures with `mockScriptRegistry.setMockScript({ sessionPurpose: "workflow-step", workflowStepTemplateId: "browser-verification" }, customScript)`. The seeded prompt body is unchanged; this behavior is provider-layer routing only.
-
 The **Frontend UX Design** template verifies visual polish and consistency with existing UI patterns and design tokens, including visual hierarchy, spacing/typography consistency, color/token consistency, component reuse, responsive behavior, and fit with existing design language.
 
 > **FN-3906 + FN-4343 auto-skip behavior:** The pre-merge orchestrator auto-skips the built-in `frontend-ux-design` step before pause/defer checks when workflow relevance signals show no frontend/UI scope. It now evaluates both (1) the task diff scope and (2) declared `## File Scope` from `PROMPT.md`. Scope relevance includes extensions (`.tsx`, `.jsx`, `.vue`, `.svelte`, `.astro`, `.html`, `.css`, `.scss`, `.sass`, `.less`, `.styl`), common UI path segments (`/components/`, `/app/components/`, `/dashboard/`, `/frontend/`, `/ui/`, `/styles/`, `/themes/`, `/design-system/`, `/design-tokens/`), and token/theme filenames (`tokens.(ts|js|json|css)`, `theme.(ts|js|json|css)`). If both signals are empty (or capture fails), Fusion preserves legacy behavior and runs the step.