adapters: amp session sync

[fosskar]

Add an amp adapter: one Adapter impl + adapter_for arm (crates/ssync-adapters).

Session storage location and format not yet investigated — needs the pi-style format investigation first (docs/pi-format-notes.md as template): layout, identity, append-only determination. amp threads may be partly server-side; determine what exists locally and is worth syncing.

If sessions live in a single database, this falls under the DB-stored-agents adapter-model question instead.

Refs: TODO.md Open #2, docs/DECISIONS.md §2.

[fosskar]

Investigated (see PR #21): amp keeps no local transcripts — threads live server-side on ampcode.com (PostgreSQL, per ampcode.com/security); local files are only CLI prompt history (history.jsonl) and UI state (session.json), confirmed on a live install. There is nothing to sync, so the adapter is declined. If amp ever ships local-first thread storage, reopen.