Rename MCP server identifier from state-hub to dev-hub

Introduce canonical MCP_SERVER_NAME constants, shared registration helpers,
and a migrate_mcp_config.py script for ~/.claude.json upgrades. Registration,
patch, and custodian CLI checks accept both dev-hub and legacy state-hub during
transition. API root metadata and session-protocol template reflect the new name.

api/main.py

@@ -136,4 +136,4 @@ app.include_router(policy.router)
 
 @app.get("/", include_in_schema=False)
 async def root():
-    return {"service": "state-hub", "docs": "/docs"}
+    return {"service": "dev-hub", "docs": "/docs"}