Wire SQLite memory into agent loop

[bug-ops]

Description

Integrate SQLite store into the agent: save messages, load history, include in context.

Parent: #4

Branch

feat/m3/memory-integration

Files

• crates/zeph-core/src/agent.rs — add SqliteStore, save/load
• crates/zeph-core/src/context.rs — include history messages
• crates/zeph-core/Cargo.toml — add zeph-memory dep
• src/main.rs — init SqliteStore, pass to Agent

Acceptance Criteria

• Each message (user + assistant) saved to SQLite
• On restart, last conversation history loaded
• History limit from config (default: 50)
• Context includes history before user message

Depends On

#17 (SQLite memory store), #16 (Shell execution — M2 complete)