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.

scripts/project_rules/session-protocol.template

@@ -1,6 +1,7 @@
 ## Session Protocol
 
-State Hub: http://127.0.0.1:8000
+Dev Hub (State Hub API): http://127.0.0.1:8000
+MCP server name in `~/.claude.json`: `dev-hub`
 
 **Step 1 — Orient**