docs: add MCP Apps (SEP-1865) documentation

Integrate comprehensive documentation for MCP Apps, the first official
MCP extension enabling interactive UI delivery.

Changes:
- guide/architecture.md (656): New section "MCP Extensions: Apps"
  - Technical architecture (primitives, SDK, security)
  - Platform support (Claude Desktop, VS Code, ChatGPT, Goose)
  - Example implementations (9 production tools at launch)
  - Developer workflow and SDK usage
  - ~150 lines of technical documentation

- guide/ultimate-guide.md (6509): New section "MCP Evolution: Apps"
  - User context and use cases
  - Available interactive tools (Asana, Slack, Figma, etc.)
  - Platform support matrix
  - Hybrid workflow examples
  - ~90 lines of user-facing documentation

- guide/ultimate-guide.md (7522): Table update
  - Added "Interactive UI" row to Plugin vs. MCP Server comparison
  - Clarified MCP Apps = "What Claude can show"

- machine-readable/reference.yaml: 8 new entries
  - mcp_apps_architecture, mcp_apps_evolution
  - Links to spec, SDK, blog posts
  - CLI relevance note (indirect)

- docs/resource-evaluations/mcp-apps-announcement.md: New evaluation
  - Score: 4/5 (High Value - Integrate within 1 week)
  - Fact-checked with Perplexity searches
  - Technical review by agent

Resource evaluated:
- https://blog.modelcontextprotocol.io/posts/2026-01-26-mcp-apps/
- https://claude.com/blog/interactive-tools-in-claude

Total documentation: ~240 lines across 3 files
Score: 4/5 (High Value)
CLI relevance: Indirect (ecosystem understanding, MCP server dev, hybrid workflows)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

machine-readable/reference.yaml

@@ -94,7 +94,16 @@ deep_dive:
   system_prompts_willison_analysis: "https://simonwillison.net/2025/May/25/claude-4-system-prompt/"
   system_prompts_prompthub: "https://www.prompthub.us/blog/an-analysis-of-the-claude-4-system-prompt"
   system_prompts_architecture: "guide/architecture.md:299"
-  # Main guide (guide/ultimate-guide.md) - Updated 2026-01-20
+  # MCP Apps Extension (SEP-1865) - Added 2026-01-27
+  mcp_apps_architecture: "guide/architecture.md:656"
+  mcp_apps_evolution: "guide/ultimate-guide.md:6509"
+  mcp_apps_spec: "https://github.com/modelcontextprotocol/ext-apps"
+  mcp_apps_blog_mcp: "https://blog.modelcontextprotocol.io/posts/2026-01-26-mcp-apps/"
+  mcp_apps_blog_claude: "https://claude.com/blog/interactive-tools-in-claude"
+  mcp_apps_sdk: "https://www.npmjs.com/package/@modelcontextprotocol/ext-apps"
+  mcp_apps_announcement_date: "2026-01-26"
+  mcp_apps_cli_relevance: "Indirect (ecosystem understanding, MCP server dev, hybrid workflows)"
+  # Main guide (guide/ultimate-guide.md) - Updated 2026-01-27
   installation: 196
   first_workflow: 277
   essential_commands: 326