I can refresh the Explorer when it's out of date

[a-team-app]

Context

The Explorer reads a folder's children once, when it's expanded, so after a git switch, a build or a script in another terminal it still lists deleted files and misses new ones, with no way to fix it short of collapsing the folder or restarting. This slice gives the user a dependable manual refresh. Pitch: #292.

Acceptance criteria

• Delete a file and create another in an expanded folder from another terminal, then run Refresh explorer from the command palette (Ctrl+E): the tree shows the new file and no longer shows the deleted one.
• The same works from the mnemonic leader: Ctrl+Space then re.
• Changes in a nested expanded folder (not just the top one) show up after one refresh.
• Every folder that was expanded before the refresh is still expanded after it; collapsed folders stay collapsed.
• The selection stays on the same file after a refresh. If that file was deleted, the selection moves to its neighbour, as it does after Delete in the Explorer.
• The tree doesn't scroll unless it has to keep the selection visible.
• If an expanded folder was itself deleted, the refresh drops it from the tree without an error.
• Refresh explorer appears in Settings › Keyboard Shortcuts with no default key, and a key bound to it there works.

Tests

• The command re-reads every expanded folder: added, removed and renamed entries, at the root and nested.
• Expansion state preserved across a refresh; collapsed folders not re-read.
• Selection kept by path; falls to the neighbour when its entry is gone; handled when the selected folder or an expanded ancestor was deleted.
• Mnemonic re registered and the prefix-free guard still passes.
• Command has no default binding.

Out of scope

• Watching folders or refreshing automatically (the next task).
• Refreshing open tabs; they already notice disk changes (A tab tells you the moment its file changes on disk #268, A clean tab shows what is actually on disk #269, A deleted file doesn't vanish from under you #271).
• A toolbar button, a default key, or a status bar message.
• Git status badges in the tree.

[a-team-app]

Draft PR #345 is up.