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>
The agent_pane_min_width config value was accepted in the schema and
passed as CapacityConfig.agentPaneWidth but never actually used — the
underscore-prefixed _config parameter in decideSpawnActions was unused,
and all split/capacity calculations used the hardcoded MIN_PANE_WIDTH.
Now decideSpawnActions, canSplitPane, isSplittableAtCount,
findMinimalEvictions, and calculateCapacity all accept and use the
configured minimum pane width, falling back to the default (52) when
not provided.
Closes#1781
