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/register-mcp.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 STATE_HUB_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 CLAUDE_JSON="${CLAUDE_JSON:-$HOME/.claude.json}"
-SERVER_NAME="${STATE_HUB_MCP_NAME:-state-hub}"
+SERVER_NAME="${STATE_HUB_MCP_NAME:-dev-hub}"
 API_BASE="${API_BASE:-}"
 MCP_URL="${MCP_URL:-}"
 DRY_RUN=0