fix: CatAgent mcpPromptMode capability — stop injecting dead MCP docs

[bouillipx]

Problem

CatAgent has mcpSupport: true but doesn't actually support native MCP or HTTP callbacks:

• mcpSupport: true → mcpAvailable = true → S13 MCP_TOOLS_SECTION injected → CatAgent sees cat_cafe_search_evidence etc. but can't call them (no MCP client)
• Setting mcpSupport: false → mcpAvailable = false → needsMcpInjection = true → C1 HTTP callback docs injected → CatAgent L0/L1 has no shell/curl to use them

Both states produce dead prompt documentation.

Solution (Design Gate passed by @codex)

Phase 1 (this issue): Route capability cleanup

Add mcpPromptMode() capability to AgentService interface:

• 'native-mcp' — inject S13 MCP docs (Claude)
• 'http-callback' — inject C1 HTTP callback (Codex/Gemini)
• 'none' — inject neither (CatAgent pre-bridge)

Changes:

• types.ts: add mcpPromptMode?() to AgentService
• CatAgentService.ts: return 'none'
• route-serial.ts + route-parallel.ts: use centralized resolveMcpPromptInjection() helper
• McpPromptInjector.ts: update to accept mode enum
• Tests: verify CatAgent gets neither S13 nor C1 in both serial/parallel

Future phases (separate issues)

• Phase 2a: Direct internal capability tools (cat_cafe_search_evidence etc. as CatAgent native tools)
• Phase 2b: Cat-cafe MCP bridge (full MCP protocol)
• Phase 3: Skill prompt loader
• Phase 4: Restore mcpSupport: true semantic

Evidence

• McpPromptInjector.ts:40-43: needsMcpInjection = !mcpAvailable
• SystemPromptBuilder.ts:607-608: if (mcpAvailable) { lines.push(MCP_TOOLS_SECTION) }
• route-serial.ts:581: mcpAvailable = (mcpSupport ?? false) && !!mcpServerPath

Verification

• CatAgent serial invocation prompt does NOT contain cat_cafe_search_evidence
• CatAgent serial invocation prompt does NOT contain HTTP 回调
• CatAgent parallel invocation prompt does NOT contain either
• Non-CatAgent providers unchanged (zero back-compat break)

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com