feat(acp): multi-session support with LRU eviction and session persistence

[bug-ops]

Summary

• Configurable max_sessions (default 4) with LRU eviction of idle sessions
• Background idle reaper task (60s tick) removes sessions past session_idle_timeout_secs (default 30 min)
• Session persistence to SQLite (acp_sessions + acp_session_events tables)
• load_session replays conversation history as session/update notifications per ACP spec
• Evicted sessions remain loadable from SQLite
• Database errors not leaked to ACP clients

Changes

File	Change
crates/zeph-core/src/config/types.rs	max_sessions, session_idle_timeout_secs in AcpConfig
crates/zeph-core/src/config/env.rs	ZEPH_ACP_MAX_SESSIONS, ZEPH_ACP_SESSION_IDLE_TIMEOUT_SECS
crates/zeph-acp/src/agent.rs	LRU eviction, idle reaper, persistence, load_session replay
crates/zeph-acp/src/transport.rs	Config wiring through AcpServerConfig
crates/zeph-acp/Cargo.toml	zeph-memory dependency
crates/zeph-memory/migrations/013_acp_sessions.sql	New tables
crates/zeph-memory/src/sqlite/acp_sessions.rs	CRUD methods for ACP sessions
src/main.rs, src/init.rs	Wire store and config

Test plan

• 2570 tests passed, 0 failed (+16 new)
• cargo clippy --workspace -- -D warnings: 0 warnings
• cargo +nightly fmt --check: clean
• LRU eviction tests (oldest idle evicted, all-busy rejects)
• SQLite CRUD tests (create, save events, load, delete, exists)
• Config snapshot updated with new fields

Deferred items (tracked)

• Session ownership check for load_session (SEC-01) — N/A until HTTP transport
• Rate limiting on new_session (SEC-02) — low risk on stdio
• SQLite growth cap (SEC-03) — acceptable at max_sessions=4
• Batch event persistence (PERF-01) — post-MVP optimization

Closes #781, closes #782
Part of Epic #778