fix(mcp): honor CONFIG_PATH env var instead of clobbering it with the --config default (#1736)

[Anai-Guo]

Summary

Fixes #1736. The documented CONFIG_PATH env-var pattern for pointing the MCP server at a custom config.yaml in container deployments was silently ignored on every boot.

Root cause

The --config CLI argument carried a non-empty Path default:

default_config = Path(__file__).parent.parent / 'config' / 'config.yaml'
parser.add_argument('--config', type=Path, default=default_config, ...)

Since that default is always truthy, args.config is populated even when --config is never passed. The startup code then ran unconditionally:

if args.config:
    os.environ['CONFIG_PATH'] = str(args.config)

so the bundled demo config path always overwrote whatever CONFIG_PATH the deployment environment had set, before GraphitiConfig() (and its YamlSettingsSource, which reads os.environ.get('CONFIG_PATH', 'config/config.yaml')) ever ran. Net effect: the env-var path never took effect, with no error or warning.

Fix

• Change the --config default to None so an explicitly-passed flag can be distinguished from the default.
• Only write CONFIG_PATH when --config was actually passed; otherwise fall back to the bundled default only if CONFIG_PATH is not already set in the environment.

Precedence is now: explicit --config > CONFIG_PATH env var > bundled default. This restores the documented env-var behavior while preserving the previous no-argument default (the bundled absolute path), so nothing regresses for users who rely on neither.

Testing

• python -m ast parse check on the modified module.
• Traced the three precedence paths against config/schema.py:303 (os.environ.get('CONFIG_PATH', 'config/config.yaml')).

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