docs(sync): merge the commit handoff into the staging tradeoff curve

The bench phase the handoff was written for is closed (splice benched,
fast-seek landed, cache removed, decision made), so the note's live half —
the firmware plumbing plan — moves into sync-commit-staging.md as "The fix —
wiring the O(depth) splice into the firmware", reconciled to the run-5 state.
The duplicated TL;DR/measurement summaries are dropped in favor of the
curve's own trail; inbound references (notes index, git_bench comments)
now point at the curve.

docs/notes/README.md

@@ -12,4 +12,3 @@
 | [`git-sync-images-and-repo-size.md`](git-sync-images-and-repo-size.md) | Why we don't shrink the notes repo — its 153 MB of media is remanso's image CDN, so rewriting history to slim the on-device clone breaks the web app. |
 | [`boot-time-budget.md`](boot-time-budget.md) | Where the ~4.3 s to cursor goes — and why the ≤ 3 s v1.0 target is hard: one ~1.9 s full refresh is unavoidable at cold boot, so the splash is nearly free. |
 | [`sync-latency.md`](sync-latency.md) | Where the ~16 s cold `:sync` goes — Wi-Fi + SNTP + one TLS push; why optimistic-retry cut a whole handshake, and why the rest is close to the protocol floor. |
-| [`sync-commit-handoff.md`](sync-commit-handoff.md) | **Handoff:** the on-device commit must become an O(depth) TreeBuilder walk — `add_all`/`index.write` re-hash the whole 570 MB-pack tree (611 s / OOM) and the real repo has never synced. Design + firmware plumbing + bench steps for the next session. |