v0.8 [2/4]: Service installer — LaunchAgent + systemd-user

[ashu17706]

Part of #71. The "just runs in the background" promise. Without this, the daemon is a manual process and the headline UX falls apart.

Scope

`src/daemon/install.ts` with two OS-specific paths:

macOS

• Generate `~/Library/LaunchAgents/dev.zero8.smriti.plist` from a template
  • `ProgramArguments`: absolute path to the smriti binary + `daemon` arg
  • `RunAtLoad: true`, `KeepAlive: true` (restart on crash)
  • `StandardOutPath` / `StandardErrorPath` pointing to `~/.cache/smriti/daemon.log`
• Register via `launchctl bootstrap gui/$(id -u) `, fall back to `launchctl load -w` for older macOS
• Uninstall reverses: `launchctl bootout` (or `unload`), remove plist

Linux

• Generate `~/.config/systemd/user/smriti.service` from a template
  • `ExecStart`: absolute path + `daemon`
  • `Restart=on-failure`
  • `WantedBy=default.target`
• Register via `systemctl --user daemon-reload && systemctl --user enable --now smriti`
• Uninstall: `systemctl --user disable --now smriti`, remove unit file

Cross-cutting

• Idempotent: running install twice produces the same registered service, no duplicates
• Detect already-installed state; output "already installed, daemon is running" without rewriting files unless --force
• Resolve smriti binary path correctly whether installed via brew, pnpm, or run from source

Acceptance

• Fresh macOS: `smriti daemon install` → reboot → daemon is running (verified via `launchctl list | grep smriti`)
• Fresh Linux: same with `systemctl --user status smriti`
• `smriti daemon uninstall` followed by reboot → daemon is not running, no orphaned plist/service file, no `~/.cache/smriti/daemon.sock`

Out of scope

• The daemon process itself (v0.8 [1/4]: Daemon core — server, watcher, queue, handlers #72)
• The user-facing CLI commands (v0.8 [3/4]: CLI subcommands + hook template #74) — install.ts is invoked from those
• Windows (deferred to a later release)