refactor(cli): migrate to commander framework and simplify config loading

Replace hand-rolled CLI argument parsing with Commander for command
registration and option handling. Streamline config resolution by
removing the BrickConfig type and accepting a plain source string
with BRICK_SOURCE env var support.

- Replace manual CLI dispatch with Commander-based command tree
- Simplify resolveConfig/resolveSource to a single resolveSource function
- Remove BrickConfig interface in favor of direct source string
- Move isRemote check into index-loader as a local utility
- Update loadIndex signature to accept source string directly

package.json

@@ -32,6 +32,7 @@
   },
   "license": "MIT",
   "dependencies": {
+    "commander": "^15.0.0",
     "js-yaml": "^5.0.0"
   }
 }