feat(tui): add /snapshots command to list and restore captured snapshots

[dgageot]

Adds a new /snapshots slash command that opens a dialog showing every snapshot captured during the current session and lets the user restore any of them (or the original pre-agent state).

What it does

• /snapshots opens a modal listing the captured snapshots with their file counts, plus an <original> entry for the pre-agent state.
• Inside the dialog: ↑/↓ (or j/k, g/G, home/end) navigate, r restores the highlighted snapshot, Esc closes without changing anything.
• Restoring snapshot N reverts every snapshot taken after N (i.e. workspace returns to the state at the end of snapshot N). Restoring <original> reverts everything.
• The command (and /undo) are hidden from both the palette and the slash-command parser when snapshots aren't enabled, so the UI stays uncluttered for users who haven't opted in.

Implementation

• Adds ListSnapshots and ResetSnapshot operations alongside the existing UndoLastSnapshot, plumbed through pkg/hooks/builtins, pkg/runtime, and pkg/app.
• A single snapshotRuntime interface in pkg/app captures all four runtime methods (SnapshotsEnabled, undo, list, reset); runtimes that don't support snapshots simply don't implement it and the related commands disappear automatically.
• popHistoryTail (renamed from truncateAfter) clears popped slots in the backing array so the dropped file lists can be GC'd.
• The dialog itself stays small: no key-map struct, no second pass over the renderer, the body and help-keys derive from len(fileCounts).

Docs & tests

• docs/features/tui/index.md updated.
• New tests cover ListSnapshots, ResetSnapshot, the no-op cases, the disabled-snapshots filtering of /undo and /snapshots, and that App.SnapshotsEnabled() answers the runtime-capability question without needing an active session.

[docker-agent]

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