## Why
Fresh startup could use saved client model and reasoning settings that differ from the app server's effective configuration. A cleared server model could also leave startup using a stale client model from bootstrap.
## What changed
- Reuse the new-session defaults reader at startup to load `model` and `model_reasoning_effort` through `config/read` before `thread/start`, using the appropriate embedded or remote working directory.
- Preserve explicit CLI and selected-profile settings, then apply managed new-thread defaults with the existing precedence rules.
- When the server has no configured model, select the startup model from its model catalog, falling back to the bootstrap default if the catalog is empty.
- Fall back to local defaults when older servers do not support `config/read`; stop startup before thread creation on other read failures.
## Testing
Add request-level tests for defaults and override precedence, working-directory selection, cleared-model fallback, request ordering, unsupported servers, and read failures.
GitOrigin-RevId: e094ba5d5501fdb34c393a0117b34dd93d8d4f2b
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