feat(ssa): finalize strict stepwise agent execution flow

Align Agent mode to strict stepwise generation and execution, add deterministic and safety hardening, and sync deployment/module documentation for Phase 5A.5/5B/5C rollout.

- implement strict stepwise execution path and dependency short-circuiting
- persist step-level errors/results and stream step_* progress events
- add agent plan params patch route and schema/migration support
- improve R sanitizer/security checks and step result rendering in workspace
- update SSA module guide and deployment change checklist

Made-with: Cursor

backend/prisma/migrations/20260311_add_ssa_agent_step_seed_fields/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "ssa_schema"."ssa_agent_executions" ADD COLUMN     "current_step" INTEGER,
+ADD COLUMN     "seed_audit" JSONB,
+ADD COLUMN     "step_results" JSONB;