chore: gitignore .mcp.json

`.mcp.json` is per-machine MCP server config (e.g. the GitHub MCP block
added during today's session). It references local env vars for auth
rather than embedding secrets, but the file itself is workstation-specific
— what servers a contributor connects depends on which MCPs they've set
up locally.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -249,3 +249,6 @@ graphify-out/
 
 # remember skill runtime state (hook logs, PIDs)
 .remember/
+
+# MCP server config (per-machine, references local env vars for auth)
+.mcp.json