Support nested $overrides entries with __self and dependency remapping

[Copilot]

$overrides could not express “override service + override its dependencies” in one node because object keys must be unique. This change adds nested __self support so a service alias and its internal dependency overrides can be declared together.

• Override resolution semantics

  • Extend Overrides shape to support nested __self.
  • Update pickOverriddenName() to:
    • resolve terminal service names from __self
    • keep traversing the matching nested override branch when a service was selected via __self, so child dependency overrides apply correctly.

• Coverage additions

  • Add unit cases in src/overrides.test.ts for:
    • direct nested __self resolution
    • parent-path nested __self resolution
    • dependency remapping through a __self-selected service
  • Add integration case in src/index.test.ts validating runtime behavior for service alias + dependency override in the same nested node.

constant('$overrides', {
  sendApplicationMessage: {
    __self: 'sendSlackMessage',
    SLACK_CONFIG: 'SLACK_APPLICATION_CONFIG',
  },
});

[nfroidure]

Closing for #145