refactor: remove the Process Engine remnants (unmounted docs router, config/process-docs, process/ components, manual process YAMLs)

[vybe]

Summary

The Process Engine was removed, but four pieces of it are still in the tree and still absorbing maintenance: a backend router that is never mounted, the documentation tree that only that router served, three Vue components nothing imports, and four manual test process definitions. None of it is reachable from the running product. Remove the cluster in one PR.

Context

Found by the forgotten-areas archaeology scan (static reachability + git last-touch). The evidence that this is dead, not merely cold:

Item	Evidence
src/backend/routers/docs.py (125 lines)	Never passed to include_router in src/backend/main.py. Its last change was the auth hardening in #452 / #507 (April) — a fix applied to endpoints nobody can reach.
config/process-docs/ (13 markdown files in getting-started/, patterns/, reference/, tutorials/)	Served only by that router; untouched since 2026-01-19. grep -r process-docs src/ hits only the unmounted router.
src/frontend/src/components/process/ — RoleMatrix.vue (420), TemplateSelector.vue (304), TrendChart.vue (182)	No import anywhere under src/frontend/src. Still received the display-label (#1643) and fetch-state (#1926) sweeps in July/August, i.e. effort keeps landing on unreachable code.
tests/manual/*.yaml (4 process definitions: compensation, gateway, notification)	Untouched since 2026-01-15; nothing under tests/ or scripts/ reads them.

Acceptance Criteria

• src/backend/routers/docs.py deleted; no include_router or import references remain (grep -rn "routers.docs\|docs_router" src/backend is empty)
• config/process-docs/ deleted, and any Dockerfile/compose COPY/mount of it removed (verify with grep -rn process-docs docker/ docker-compose*.yml scripts/)
• src/frontend/src/components/process/ deleted; npm run build and npm run test:unit pass
• tests/manual/ process YAMLs deleted (or the whole directory if nothing else lives there)
• Feature-flow docs that describe this surface are archived rather than left pointing at deleted files: docs/memory/feature-flows/execution-list-page.md (references the process-engine executions repository and is not linked from the index) and the process-engine references in tasks-tab.md, parallel-headless-execution.md (useProcessWebSocket.js), testing-agents.md (tests/process_engine/conftest.py)
• docs/memory/architecture.md router catalog no longer lists docs.py
• Unit + frontend suites green; no new raw-color or loading-gate ratchet regressions (deletions only)

Technical Notes

• Deletions only — no behaviour change is expected, so the PR should be reviewable by diff stat alone.
• Before deleting each Vue component, confirm there is no dynamic consumer (defineAsyncComponent, <component :is>, string-built import paths); the reachability scan is a word-level search and cannot see those.
• The three sibling findings from the same scan are filed separately: unreferenced Vue components outside process/, dead backend modules, and the documentation drift sweep.