Accept spike 2: MCP HTTP endpoint for workflow and agent tools

Adds POST /mcp endpoint speaking MCP Streamable HTTP (JSON-RPC 2.0)
with 12 tools for workflow management and agent orchestration.
Supports both JSON and SSE response modes. Includes real-time agent
output streaming over SSE, Content-Type validation, and 15 integration
tests (134 total).

Tools: create_story, validate_stories, list_upcoming, get_story_todos,
record_tests, ensure_acceptance, start_agent, stop_agent, list_agents,
get_agent_config, reload_agent_config, get_agent_output.

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

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "story-kit": {
+      "type": "http",
+      "url": "http://localhost:3001/mcp"
+    }
+  }
+}