fix(coding-agent): allow Luna children on empty Codex catalogs

[cskwork]

Summary

Allow a Sol parent to discover and explicitly spawn a GPT-5.6 Luna RLM child without pinning Prime Agent to an unrelated Codex CLI version.

The /codex/models endpoint currently returns an empty successful catalog when Prime Agent sends its own client_version. An empty catalog is not proof that the authenticated account has no usable models: direct Luna requests work with the same credentials.

This change:

• treats an empty successful Codex catalog as inconclusive and keeps locally known Codex models executable;
• keeps non-empty account catalogs authoritative, preserving their filtering;
• keeps authentication preflight unchanged;
• adds an end-to-end regression proving a Sol parent discovers, admits, and retains a Luna child when discovery returns an empty catalog.

Fixes #1011. This overlaps the empty-catalog policy in #696/#717 but adds the exact Sol-to-Luna RLM regression.

Verification

• Focused tests: 13/13 passed
• npm run check: passed
• Real patched-source run against the live service:
  • Sol parent
  • rlm.find_models("luna") returned openai-codex/gpt-5.6-luna
  • spawn handle reported openai-codex/gpt-5.6-luna
  • real child returned NO_PIN_LUNA_OK

Please merge this fix (or fold its Sol-to-Luna regression into #717) so explicit Luna children work without maintaining a hard-coded external CLI version.

Note

Fix Luna child model discovery when Codex model catalog returns empty

• In model-registry.ts, introduces a filterByCatalog helper that treats an empty catalog as non-authoritative, leaving all local openai-codex models available instead of filtering them all out.
• Previously, an empty catalog response (e.g. due to a backend gate) would cause all models to be filtered, breaking Luna subagent discovery.
• Adds a regression test in 1011-luna-subagent-selector.test.ts that stubs an empty catalog fetch and asserts Luna is discoverable and executes successfully.

^{Macroscope summarized b72703c.}

[sethkarten]

Closed as superseded by #1153, an independent implementation written from upstream/main without importing outside commits, source code, or test files. #1153 resolves the Sol-to-Luna child-selection path with its own implementation in the existing regression suite.