Refactor history database migrations

[coder-hhx]

Summary

• centralize chat-history.sqlite3 connection setup and schema migrations in a shared history_db module
• migrate chat history, FTS metadata, subagent history, and message bus schema through PRAGMA user_version
• run history DB initialization during app setup before services can issue history commands

Verification

• cargo test --manifest-path crates/agent-gui/src-tauri/Cargo.toml history -- --nocapture
• cargo build --manifest-path crates/agent-gui/src-tauri/Cargo.toml
• pnpm --dir crates/agent-gui build
• pnpm --dir crates/agent-gui tauri build