Raise default CI batch timeout from 3m to 15m (#750)

## Summary
- Bumps the default `[ci].batch_timeout` from 3 minutes to 15 minutes in
`CIConfig.ResolvedBatchTimeout`.
- The 3m default expired before slower agents (codex, gemini) could
finish, causing PR comments with partial counters (e.g. `1+0/3`) and
discarded output from still-running child processes.
- Updates the doc comment on the `BatchTimeout` field and the unit tests
that assert the default.