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/schema.prisma

@@ -2549,6 +2549,9 @@ model SsaAgentExecution {
   reviewResult    Json?    @map("review_result")
   executionResult Json?    @map("execution_result")
   reportBlocks    Json?    @map("report_blocks")
+  stepResults     Json?    @map("step_results")
+  currentStep     Int?     @map("current_step")
+  seedAudit       Json?    @map("seed_audit")
   retryCount      Int      @default(0) @map("retry_count")
   status          String   @default("pending")      /// pending | planning | coding | reviewing | executing | completed | error
   errorMessage    String?  @map("error_message")