adapters: Claude Code session sync

[fosskar]

Add a Claude Code adapter: one Adapter impl + adapter_for arm (crates/ssync-adapters).

Sessions live under ~/.claude/projects/**. Before trusting store-as-is, the format needs the same investigation pi got (docs/pi-format-notes.md is the template): per-session file layout, session identity, append-only determination, whether identity can be derived from path/filename alone.

If sessions turn out to live in a single database instead of per-session files, this falls under the DB-stored-agents adapter-model question instead (see that issue).

Refs: TODO.md Open #2, docs/DECISIONS.md §2 (store as opaque bytes), AGENTS.md adapter recipe.

[fosskar]

Adapter landed in PR #21 (claude-code, root ~/.claude/projects). Ships append_only() = false: upstream docs claim append-only (compaction/resume/rename all append; anthropics/claude-code docs/en/sessions.md, docs/en/agent-sdk/session-storage.md), but no real session file has been diffed across a compaction/rewind cycle, and a wrong true scrambles content on conflict. Until then conflicts are newest-wins.

Remaining before flipping the merge gate on: inspect a real ~/.claude/projects/**/<uuid>.jsonl through a /compact and a rewind, confirm the file only ever grows. Criteria in docs/claude-code-format-notes.md.

[fosskar]

Partial real-file verification done (Claude Code 2.1.205, unauthenticated claude -p still writes transcripts):

• Layout + identity confirmed against a real file: /tmp/ssync-probe → ~/.claude/projects/-tmp-ssync-probe/<uuid>.jsonl, filename = session uuid, verbatim cwd inside.
• Resume = append proven: claude -p --resume <id> grew the same file by ~3.5KB; prior 14,179 bytes byte-identical (cmp -n prefix check).
• Caveats found: line 1 is not always the documented system/init event (saw queue-operation first), and the project dir can contain non-session entries (memory/).

Still needed to flip append_only(): an authed session driven through /compact and a rewind, then the same prefix diff. Codex (#7) could not be exercised at all — CLI unauthenticated (401), failed runs write no rollout file.

[fosskar]

Precision correction to the previous comment: the resumed turn was the CLI's synthetic auth-error exchange (\"model\":\"<synthetic>\", "Not logged in"), since the run was unauthenticated. So the proven statement is: the transcript writer appends to the same file on resume, even in the error flow — layout/identity confirmation stands. Real conversational growth, /compact, and rewind remain unverified and all need an authed session before the merge gate flips.