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

[ranxianglei]

Protocol-consolidation Phase P (user direction m00509: billion-context 的持久化机制进入内核，下游复用).

What

• SessionStore becomes a thin POLICY adapter over StateStore<PersistedSession> (acp-kernel/persist, shipped 0.0.40/fix(anthropic): forward round-2 stream with proper framing (vertical-text bug) #126 + 0.0.41/release v0.1.38 #130): ~270 lines of duplicated mechanism deleted (atomic write, rename retry, debounce timers, write chains, corrupt-tolerant walk).
• Kernel gains nothing proxy-specific; proxy keeps: record schema (PersistedSession v3), namespaced layout (relPathFor), validity, build/buildSession, env knobs (BILI_PERSIST*), 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, incl. v1) load via the kernel's legacy adoption hook and re-persist as envelope on the next dirty write — old sessions never break, files migrate organically.

Semantics upgrades (documented in tests)

• scheduleSave builds the record at WRITE time (freshest state under coalescing) instead of capturing at call time.
• In-process loadSync with stale meta still resolves via the kernel's discovered-file cache — id is authoritative, meta is only a path hint (isolation probe uses a cold store).

Deps

• acp-kernel 0.0.41 (carries the fix: re-inject the client's openai system around the kernel hoist (0.0.37) #193 hoist + persist subpath; base is fix: re-inject the client's openai system around the kernel hoist (0.0.37) #193 — merge that first).

Tests

• 513/513 green (512 baseline + 1: pre-envelope flat-file adoption → re-persist envelope round-trip). tsc build-scope clean. Fixes nothing user-visible on its own; unlocks single-sourced persistence across proxy + omp (billion-context-omp#131).

[ranxianglei]

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