Setting to disable automatic session focus switching when an agent needs attention

### Feature summary

Add a setting to disable automatic session focus switching, so the app never pulls a background session into the foreground when it "needs attention" (ready for input, finished, etc.).

### What problem are you trying to solve?

I usually have many sessions running in parallel. While I'm reading or actively typing in one session, the app frequently snaps the UI over to a different session the moment one of the background agents needs attention (e.g. a tool call requires input, a plan needs approval, or a session finishes). This is disruptive: it interrupts my typing, makes me lose my place, and pulls me away from the session I deliberately chose to focus on.

I don't need the app to route me automatically, because the sidebar already shows per-session status icons (needs a question answered, done, etc.). I can see at a glance which sessions need me and switch to them on my own terms.

### Proposed solution

A persistent preference (app settings, e.g. under **General**) to control automatic focus switching. Something like:

- **Auto-switch to sessions that need attention** — On / Off (default On to preserve current behavior).

When set to Off:
- The foreground session never changes unless I click it myself.
- Sessions that need attention or complete are still indicated via their existing sidebar status icons (and ideally an optional unread/attention badge), but they do not steal focus.

Granularity would be a bonus (e.g. separate toggles for "switch when a session needs input" vs. "switch when a session completes"), but a single on/off toggle would solve the core problem.

### Workflow impact

This affects anyone orchestrating multiple parallel agent sessions — arguably the core use case of the app. For power users running many sessions at once, automatic focus stealing actively fights the "control center" workflow: I'm trying to drive one session while others run in the background, and the UI keeps yanking me elsewhere. An opt-out would let people who prefer to navigate manually (via the status icons) stay in control, while leaving the current auto-switch behavior as the default for those who want it.

### Additional context

Related existing issues:

- #968 — bug report: "Session switches while typing if tool call in non-foreground session requires input." That issue is the narrower typing-interruption symptom; this request is the broader, user-facing **preference toggle** to opt out of auto-switching entirely (not only while typing).
- #192 — request for attention badges, which pairs well with this: badges/icons instead of focus stealing.
- #522 — request for a keyboard shortcut to jump to the next unread session, i.e. manual, user-initiated switching.

Together these point at the same theme: let users rely on status indicators and switch sessions manually, rather than having focus moved for them.

No files, tests, or entry points are named. Start by locating the app's General settings and the existing automatic session focus-switching behavior. Trace the attention events that trigger switching, then verify that disabling the preference keeps the current session focused while existing sidebar status indicators remain available.

索引モデルが issue の本文から書いたものです。