fix(files): reload the file tree while a run writes and on window focus (#1144)

[agegr]

Fixes #1144.

Problem

The file tree reloaded only when a run ended or when you pressed refresh. Files the agent wrote during a long run, or files changed in an editor or terminal, stayed hidden until then.

Change

• useAgentSession reports each top-level tool_execution_end via a new onToolEnd option. AppShell then schedules one tree reload at most once a second, skipping the read-only preset's read/grep/find/ls. It uses the existing explorerRefreshKey, so the git status and an open diff refresh too.
• FileExplorer reloads when the window gets focus, which covers changes made outside Pi Web.
• No fs watcher and no polling. A reload keeps the open folders and shows no spinner, as the refresh button already did.
• Note in docs/agents/files-and-access.md.

Checked

• tsc --noEmit, eslint on the touched files, node --test hooks components (905 pass).
• Isolated dev server (port 30241, scratch agent dir) with headless Chrome: I created a file in the project from outside. It was not listed before focus (count 0) and was listed after a window focus (count 1).
• Not checked: the tool-end path during a real model run (no model in the scratch setup).

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