Document --dump-models-group CLI contract #2265

Summary

Keep the --dump-models-group CLI documentation aligned with its implementation.

Required changes

• Update src/lightspeed_stack.py's main() docstring to describe --dump-models-group and state that failures while dumping exit with status 1.
• Update the README.md CLI usage synopsis to include [-gr DUMP_MODELS_GROUP].

Rationale

The option is implemented, but its public usage and failure contract are not fully documented.

Affected areas

• src/lightspeed_stack.py
• README.md

Acceptance criteria

• main() documentation lists the --dump-models-group behavior.
• The documented Raises/failure behavior covers model-group dump failures and status 1.
• The README usage synopsis contains [-gr DUMP_MODELS_GROUP].

Backlinks

• Source pull request: LCORE-3141: models group schema export ability #2263
• Source review comment: LCORE-3141: models group schema export ability #2263 (comment)

Requested by: @tisnik