fix: live-refresh desktop file panel on disk changes

[acehighdan]

Issue for this PR

Closes #48451

Type of change

• Bug fix

What does this PR do?

The desktop "All files" panel did not live-refresh on disk changes made outside the agent Write tool: created files appeared only after an unrelated refresh, and deleted files never disappeared.

Root cause: the filesystem watcher is location-scoped. When a session directory is invalidated, its inotify subscription is torn down and nothing re-subscribes it, so the panel stops receiving change events.

Fix, two parts:

1. packages/core: watch the workspace directory unconditionally instead of gating it behind the experimental flag + VCS check, so non-git roots get live events too.
2. packages/app: while the window is visible, poll every expanded/loaded directory every 5s when watcher events are absent. FileSystem.list is cacheless and error toasts are suppressed for polled directories. Watcher events remain the fast path.

How did you verify your code works?

• bun typecheck clean and 45 file-context unit tests pass in packages/app.
• Reproduced on the desktop app: external touch and rm now appear/disappear in the panel within one poll cycle.
• Updated the core watcher test; confirmed it passes for non-git roots.

Screenshots / recordings

n/a (behavior change, verified live rather than visual)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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