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/migrations/20260308_default_agent_mode/migration.sql

@@ -0,0 +1,8 @@
+-- 1. 修改列默认值：新 session 默认使用 agent 模式
+ALTER TABLE "ssa_schema"."ssa_sessions"
+  ALTER COLUMN "execution_mode" SET DEFAULT 'agent';
+
+-- 2. 将所有已有 session 从 qper 更新为 agent
+UPDATE "ssa_schema"."ssa_sessions"
+  SET "execution_mode" = 'agent'
+  WHERE "execution_mode" = 'qper';