fix(config): increase default chat timeout

[owainlewis]

Summary

• Increase the default chat backend timeout from 2 minutes to 10 minutes.
• Preserve explicit run_timeout overrides.
• Update the configuration reference and example.

Why

Research and multi-tool Codex requests were repeatedly terminated at the exact 120-second default, including a request that researched AI news and created a Passage document. Ten minutes gives normal agent workflows enough time while keeping a finite bound.

Test plan

• cargo test (257 tests passed)
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check
• Independent diff review: no findings

Risks

Slow or stuck chat runs can now occupy a worker for up to 10 minutes by default. Users can set a shorter explicit run_timeout.

Related issue

None.