[Feature]: Add configuration to keep terminal splits open/visible after task completion

### Problem or use case

Currently, when an agent finishes execution or a task/worktree reaches a completed or resolved state, the UI automatically unmounts or collapses the active terminal split to switch directly to the diff/review view.

While this prioritizes diff review and PR creation, it disrupts workflows where developers need to:
- Inspect full process exit states, trailing logs, and error outputs without having to manually reopen panels.
- Interact with lingering background processes, local dev servers, or REPLs started during the task run.
- Prevent abrupt layout shifts across split-pane views when running and checking parallel worktrees.

### Proposed solution

Add a user setting and/or an `orca.yaml` configuration option to control terminal lifecycle behavior upon agent completion:

- **Config Key Examples:** `keepTerminalOpenOnComplete: true` or `terminal.autoCloseOnComplete: false`
- **Behavior:** When enabled, completing a task keeps the active terminal split pinned and interactive in the foreground alongside/below the diff review view rather than collapsing the PTY split automatically.

### Alternatives or additional context

- **Current Workaround:** Manually reopening the terminal split using `Cmd/Ctrl + \`` or clicking the bottom toolbar icon after every finished run.
- **Related Context:** Having the terminal pane remain open provides continuity when transitioning between agent execution and manual verification.

No implementation files or tests are named. Start by locating the terminal split lifecycle and the settings or orca.yaml configuration entry points, then trace what happens when an agent or worktree reaches completion. Done means an opt-in setting keeps the terminal split pinned and interactive while the diff or review view remains available.

Written by the indexing model from the issue text.