feat(rlm): configure a default subagent model

[olivermontes]

Summary

Adds subagents.defaultModel as a persistent global or project-level default for RLM child agents.

Resolution order is now:

1. Explicit rlm(..., model=...)
2. subagents.defaultModel
3. Parent-model inheritance

Configured selectors use the existing authenticated-catalog lookup and auth preflight. An unavailable selector fails admission rather than silently falling back.

Closes #921.

Validation

• npm test -- test/suite/regressions/4649-subagent-model-selection.test.ts test/settings-manager.test.ts
• npx biome check packages/coding-agent/src/core/settings-manager.ts packages/coding-agent/src/core/agent-session.ts packages/coding-agent/test/settings-manager.test.ts packages/coding-agent/test/suite/regressions/4649-subagent-model-selection.test.ts
• npm run build

Note

Add subagents.defaultModel setting to configure a default subagent model in RLM

• Adds a subagents.defaultModel field to the Settings interface and a getSubagentDefaultModel() accessor on SettingsManager.
• In AgentSession.runRlmChild, the configured default is used when no explicit model is passed to rlm.run, falling back to the parent model only if neither is set.
• Behavioral Change: if the configured default model is unavailable or fails auth preflight, spawn now fails rather than falling back to the parent model.

^{Macroscope summarized c4fe65d.}