refactor(persist): delegate SessionStore mechanism to acp-kernel/persist

[ranxianglei]

Reopen of #196 (closed defensively, not merged). Protocol-consolidation Phase P.

What

• SessionStore becomes a thin POLICY adapter over StateStore<PersistedSession> (acp-kernel/persist 0.0.41): ~270 lines of duplicated mechanism deleted (atomic write, rename retry, debounce timers, write chains, corrupt-tolerant walk).
• Proxy keeps: record schema (v3), namespaced layout (relPathFor), validity, build/buildSession, env knobs, singleton. Public API unchanged — session.ts/server.ts/export.ts untouched.

On-disk migration

• New files: envelope {version, savedAt, id, payload}. Old flat files (all versions) load via the kernel legacy adoption hook; next dirty write re-persists as envelope — old sessions never break.

Tests

• 513/513 green (512 baseline + 1 new: pre-envelope flat-file adoption → re-persist round-trip). tsc clean. Base is fix: re-inject the client's openai system around the kernel hoist (0.0.37) #193 (merge that first). MERGE BY OWNER ONLY.