Agent area width now uses real mainPane.width instead of hardcoded 50%
ratio. Grid planning, split availability, and spawn target finding now
respect user's agent_pane_min_width config instead of hardcoded
MIN_PANE_WIDTH=52, enabling 2-column grid layouts on narrower terminals.
Use the configured agent pane width consistently in split target selection and avoid close+spawn churn by replacing the oldest pane when eviction is required.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
liu-qingyuan