Add unattended sync mode plus scheduled daemon runner

[Codex]

Enabled periodic/non-interactive sync to keep SSH keys in sync without prompts. The new sync and daemon commands run uploads/downloads safely in unattended contexts, skipping conflicts instead of blocking.

• Non-interactive safety: Upload/download/sync accept --non-interactive/--quiet (or auto when stdin isn’t a TTY); conflicting local/server files are skipped with stderr notes; local-only files are left untouched.
• Sync workflow: New sync command runs upload then download in one shot; safe-mode and path flags are respected.
• Daemon scheduling: New daemon command loops sync on a configurable interval (default 1h), exiting cleanly on SIGINT/SIGTERM; defaults to non-interactive.
• Docs/tests: README documents sync/daemon usage; added coverage for non-interactive skip behavior.

Example:

# Hourly unattended reconciliation
ssh-sync daemon --interval 1h --non-interactive

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Periodic/automatic sync — schedule or daemon mode</issue_title>
<issue_description>## Overview

Allow ssh-sync to automatically run a sync on a recurring schedule so users don't have to remember to run ssh-sync sync manually.

This builds on top of the sync command proposed in #90.

Motivation

SSH keys change infrequently but the consequences of being out of sync (e.g. a key missing on a new machine, or a revoked key still present locally) can be serious. Automating periodic sync removes the manual burden and keeps all machines consistent.

Proposed approaches

Option A — OS-level scheduling (recommended for initial implementation)

Provide a helper command that installs the appropriate scheduler entry for the current platform:

ssh-sync schedule install   # installs cron/launchd/systemd unit
ssh-sync schedule remove    # removes it
ssh-sync schedule status    # shows current schedule

Platform targets:

• Linux: systemd user timer (~/.config/systemd/user/ssh-sync.{service,timer}) or a crontab entry
• macOS: launchd plist (~/Library/LaunchAgents/com.github.therealpaulgg.ssh-sync.plist)
• Windows: Task Scheduler via schtasks

The schedule interval should be configurable (default: every hour). The generated unit/plist/task should run ssh-sync sync non-interactively — which implies the sync command must handle the case where stdin is not a TTY (skip prompts, or adopt a safe default such as skipping rather than deleting on conflict).

Option B — Built-in daemon mode

Add a ssh-sync daemon sub-command that blocks and triggers a sync on a configurable interval using a Go ticker:

ssh-sync daemon --interval 1h

This is simpler to implement cross-platform but requires the process to stay running (e.g. managed by systemd/launchd anyway).

Non-interactive sync considerations

When running unattended (no TTY), the sync command must not hang waiting for user input. Suggested behaviour:

• If a server key is newer than the local key → skip (do not overwrite)
• If a local key is absent from the server → skip deletion (keep the local key)
• Log all skipped conflicts to stderr / a log file so the user can review them later

A --non-interactive / --quiet flag on sync (or auto-detected via term.IsTerminal) would control this path.

Related

• Implement ssh-sync sync command (upload + download) #90 — sync command (prerequisite)
• Conflict resolution - Add change history for ssh keys ssh-sync-server#21 — server-side conflict resolution / change history
  </issue_description>

Comments on the Issue (you are @codex[agent] in this section)

@therealpaulgg Possible inspo? Or maybe we can do better? https://blog.atuin.sh/atuin-v18-3-0/

• Fixes Periodic/automatic sync — schedule or daemon mode #91