[tim-smart]

Summary

• Set the chat bot default to openai/gpt-6-astra, preserving the endpoint's existing provider-prefix convention.
• Keep the existing medium reasoning default and environment overrides. PR review and reply-gate models are unchanged.
• Update the configuration reference and add a regression test for the model/effort defaults.

Validation

• Configuration test failed against the old default and passes with the change.
• Vite+ check: zero errors (existing warnings).
• Full Vite+ test rerun: 158 files, 1,024 tests passed. The first full run had transient failures; no additional code changes were needed for the successful rerun.
• Live endpoint model availability and production environment overrides were not verified; this PR changes the repository default, not deployed secrets.

Closes EFF-3713