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/src/modules/ssa/routes/chat.routes.ts

@@ -115,12 +115,8 @@ export default async function chatRoutes(app: FastifyInstance) {
       }
       // ── H1 结束 ──
 
-      // 3. 读取 session 的执行模式
-      const session = await (prisma.ssaSession as any).findUnique({
-        where: { id: sessionId },
-        select: { executionMode: true },
-      });
-      const executionMode = (session?.executionMode as string) || 'qper';
+      // 3. 执行模式：统一使用 Agent 通道（QPER 已废弃 UI 入口）
+      const executionMode = 'agent';
 
       // ── Agent 通道分流 ──
       if (executionMode === 'agent') {