fix(daemon): start cache daemon as user service

[tee8z]

Summary:

• Auto-start daemon-backed caching through the user service manager: systemd-run --user on Linux and launchctl LaunchAgent on macOS.
• Preserve a direct detached-spawn fallback for environments without a service manager.
• Pass socket and desktop-session environment into the daemon so editor/agent one-shot shells can reuse the same warm cache.
• Document the auto-start behavior in the daemon and agent setup docs.

Validation:

• cargo fmt --check
• cargo build --locked
• cargo test --locked
• Runtime Linux validation with a dedicated VIA_DAEMON_SOCKET: one fresh process started the daemon and completed a Linear GraphQL viewer query; a second fresh process reported via daemon: running with cached entries and completed the same query without another 1Password login prompt. ps showed the daemon parented by systemd --user.