fix: recover delegated idle streams and preserve model override policy

[aheritier]

Fixes #4232

Summary

• Recover sentinel-only pre-response delegated idle streams with a one-time direct-transfer retry.
• Make retry admission, cancellation, and budget propagation safe.
• Preserve target-provider defaults when parallel_tool_calls is omitted.
• Isolate per-agent CLI model override policy while preserving real model identity.
• Add/update tests and documentation.

Validation

• task build
• task test
• task lint
• Scoped race/shuffle tests
• Docs link check: only baseline-equivalent external 403/404 failures

The implementation is organized into these five logical commits:

1. fix(runtime): classify idle streams before output
2. fix(runtime): retry admitted delegated idle streams
3. fix(runtime): recover direct task transfers
4. fix(config): preserve omitted default model policies
5. fix(config): isolate CLI model override policies