[codex] Add agent-scoped model settings

[botlong]

Summary

• Move profile/model settings from global defaults to per-agent settings.
• Add gateway /settings/agents persistence and use agent-bound profile/default model for sessions and model listing.
• Let the settings UI choose a config for Codex, Claude Code, or OpenCode, load models through that config, and save the agent default model in the same flow.
• Fetch Codex models from the selected OpenAI profile and map OpenCode profile credentials into OpenAI-compatible environment variables.

Validation

• npm test in gateway passed: 35 tests.
• flutter analyze passed with no issues.
• flutter test passed: 15 tests.
• git diff --cached --check passed.