feat(ssa): SSA Agent mode MVP - prompt management + Phase 5A guardrails + UX enhancements

Backend:
- Agent core prompts (Planner + Coder) now loaded from PromptService with 3-tier fallback (DB -> cache -> hardcoded)
- Seed script (seed-ssa-agent-prompts.ts) for idempotent SSA_AGENT_PLANNER + SSA_AGENT_CODER setup
- SSA fallback prompts added to prompt.fallbacks.ts
- Phase 5A: XML tag extraction, defensive programming prompt, high-fidelity schema injection, AST pre-check
- Default agent mode migration + session CRUD (rename/delete) APIs
- R Docker: structured error handling (20+ patterns) + AST syntax pre-check

Frontend:
- Default agent mode (QPER toggle removed), view code fix, analysis result cards in chat
- Session history sidebar with inline rename/delete, robust plan parsing from reviewResult
- R code export wrapper for local reproducibility (package checks + data loader + polyfills)
- SSA workspace CSS updates for sidebar actions and plan display

Docs:
- SSA module doc v4.2: Prompt inventory (2 Agent active / 11 QPER archived), dev progress updated
- System overview doc v6.8: SSA Agent MVP milestone
- Deployment checklist: DB-5 (seed script) + BE-10 (prompt management)

Made-with: Cursor

backend/prisma/schema.prisma

@@ -2481,7 +2481,7 @@ model SsaSession {
   dataPayload   Json?    @map("data_payload")   /// 真实数据（仅R可见）
   dataOssKey    String?  @map("data_oss_key")   /// OSS 存储 key（大数据）
   dataProfile   Json?    @map("data_profile")   /// 🆕 Python 生成的 DataProfile（Phase 2A）
-  executionMode String   @default("qper") @map("execution_mode") /// qper | agent
+  executionMode String   @default("agent") @map("execution_mode") /// qper | agent
   status        String   @default("active")     /// active | consult | completed | error
   createdAt     DateTime @default(now()) @map("created_at")
   updatedAt     DateTime @updatedAt @map("updated_at")