feat: local version history — automatic snapshots, diff & restore

[ijbo]

What

Local version history — the classic must-have of serious editors, fully client-side. Snapshots of every workspace file are captured automatically as you edit and stored in an IndexedDB ring buffer; a History panel (right-click a file in the sidebar) shows a timeline with a GitHub-style line diff vs the current document, one-click Restore (with a pre-restore safety snapshot, so restores are undoable) and Copy.

Zero new dependencies — the line diff is ~90 lines of hand-rolled LCS with prefix/suffix trimming, matching the project's zero-dep ethos.

How it works

• Capture policy: first save of a file → snapshot; then every ≥3 min of editing, or immediately when ≥400 chars change. Identical content never re-snapshots.
• Hooks at the two authoritative save choke points: saveToLocalStorage (keystroke autosave) and setFileContent (file-switch saves). All calls guarded — the app behaves identically if the module fails to load.
• Storage: IndexedDB textagent-history, split into meta (tiny records for listing/pruning) and content (loaded on demand).

Hardened via adversarial multi-agent review (9 confirmed findings fixed pre-merge)

• meta/content store split — listing and pruning never deserialize document bodies (was: every capture re-read all snapshot contents)
• Byte budgets — 25 MB/file + 50 snapshots/file + 400 global caps; docs >5 MB are never snapshotted (quota safety)
• Diff rendering collapses unchanged runs to 3-line context and hard-caps rendered rows — no DOM blowup / tab freeze on huge documents
• Restore reads authoritative content — disk-folder and single-linked files are read from disk (new disk.readSingleFile), not the stale localStorage cache, for both the diff and the safety snapshot
• Honest failure reporting — a quota-failed restore shows an error toast instead of a false success
• File delete purges its history — "cannot be undone" now includes snapshots; no sensitive retention, no orphans evicting live history via the global cap

Testing

• New 9-test Playwright spec: API surface, IDB snapshot storage, diff correctness (adds/dels/sames exact), panel open/close, full restore round-trip incl. safety snapshot, identical-content dedupe, context-menu item, history purge.
• Verified visually at 1280×800 (timeline, delta badges, +2/−1 diffstat, GitHub-style diff, Copy/Restore).
• Smoke suite green (one pre-existing view-mode flake passes in isolation); build clean.

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