feat(core): add per-note history timeline with restore

[haribo]

Summary

Implements the note-history core (design: docs/design/note-history.md).

• note_history(id) — coalesces content-changing edits into versions (author change or ≥ 5 min idle gap), newest first, capped at 100; unchanged-content steps are skipped (which also filters out edits to other notes).
• note_at(id, version_id) — historical read via Automerge text_at/get_at/keys_at at the snapshot's heads.
• restore(id, version_id, now) — a forward, merge-safe re-apply of title/body/tags/folder; itself a new history entry.
• Each mutation now commits as its own change stamped with the caller's now. This gives per-edit history granularity and makes change timestamps deterministic — the model's "reads no clock" contract, previously only honoured at the field level, now holds at the change level too.

Test plan

• cargo test --workspace — 6 new core tests (coalescing + read-back, restore reverts + is a new version, restore is merge-safe vs a concurrent edit, 100-cap, bad version id) + no regression across sync/e2e/relay
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check

Part of #179
Closes #182