refactor(cleanup): remove legacy runtime directories and fix API semantics

Task 1: Clean up root-level runtime directories
- Backup live/, backtest/, production/ to runs/_legacy/
- Remove legacy directories from repo root

Task 2: API route semantics cleanup
- Create /api/runs/{run_id}/agents/* routes for runtime agent operations
- Keep /api/workspaces/{id}/agents/* for design-time (deprecated)
- Update frontend runtimeApi.js to use new /runs/ prefix
- Update legacy-inventory.md with completion status

New files:
- backend/api/runs.py - Runtime agent routes with proper run_id semantics

Modified:
- backend/api/__init__.py - Export runs_router
- backend/apps/agent_service.py - Include runs_router, update scope docs
- frontend/src/services/runtimeApi.js - Use /runs/ instead of /workspaces/
- docs/legacy-inventory.md - Mark cleanup as completed

Constraint: Maintain backward compatibility with old /workspaces/ routes
Rejected: Remove old routes entirely | need backward compatibility during transition
Confidence: high
Scope-risk: moderate
Directive: Old /api/workspaces/ routes remain functional but deprecated
Not-tested: Full integration test with active runtime

docs/legacy-inventory.md

@@ -51,8 +51,8 @@ in use.
 | Surface | Location | Replacement | ETA |
 |---------|----------|-------------|-----|
 | Legacy analyst agents | `backend.agents.analyst.*` | `EvoAgent` | After EvoAgent smoke tests pass |
-| Mixed workspace_id semantics | `/api/workspaces/{id}/agents/...` | Explicit `run_id` vs `workspace_id` routes | TBD |
-| Root-level runtime directories | `live/`, `backtest/`, `production/` | `runs/<run_id>/` | Already deprecated, safe to ignore |
+| Mixed workspace_id semantics | `/api/workspaces/{id}/agents/...` | ✅ `/api/runs/{run_id}/agents/...` routes added | Completed |
+| Root-level runtime directories | `live/`, `backtest/`, `production/` | `runs/<run_id>/` | ✅ Removed, backed up to runs/_legacy/ |
 
 **Status**: Do not add new code using these surfaces. Migrate existing usage
 when touching related code.