Per-session resource usage view (CPU, memory, disk)

### Feature summary

_No response_

### What problem are you trying to solve?

When I run several agent sessions at the same time, my machine sometimes runs low on CPU, memory, or disk, and sessions slow down. Today there is no way inside the app to see which session is responsible.

The operating system's own tools (Activity Monitor, Task Manager, `top`) only show individual processes. They cannot tell me which session a process belongs to, so when many sessions run in parallel each starting its own builds, servers, and tests, it is very hard to find the one that is causing the slowdown.

Disk is a particular pain point. Session working directories add up quickly, and I have no easy way to tell which sessions are using the most space so I can clean them up.

### Proposed solution

A view in the app that shows resource usage grouped by session instead of by individual process:

- CPU and memory per session, including everything that the session starts, so the numbers reflect the real cost of a session.
- Disk usage per session, so it is clear which sessions are taking up space.
- A list of all active sessions, sortable by usage, so the heaviest session is easy to find.
- An optional visual view (for example a treemap) to spot the largest consumers at a glance.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points, so first identify how the desktop app tracks sessions, their child processes, and working directories. Done should provide an active-session view with sortable per-session CPU, memory, and disk usage, including resources started by each session.

索引モデルが issue の本文から書いたものです。