#285 Docs: How-to guide for background autoupdating

Context

Issue #227 adds opt-in background autoupdating for the swamp binary with:

• swamp update --setup-auto interactive setup
• swamp update --setup-auto status to check current state
• swamp update --setup-auto disable to tear down
• swamp config set/get/list for non-interactive configuration
• Platform-native scheduling (launchd on macOS, systemd on Linux)
• Autoupdate logging at ~/.swamp/log/autoupdate.log

Requested documentation

A how-to guide at content/manual/how-to/set-up-background-autoupdating.md covering:

1. Enabling autoupdate interactively (swamp update --setup-auto)
2. Enabling autoupdate non-interactively (swamp config set update.auto enabled)
3. Choosing a cadence (daily vs weekly)
4. Checking autoupdate status
5. Disabling autoupdate and removing the scheduler
6. Viewing the autoupdate log
7. How it works under the hood (launchd on macOS, systemd on Linux)
8. CI/non-interactive environments (autoupdate is opt-in, never auto-enabled)

Additionally, reference documentation should be added or updated for:

• swamp update command (new flags: --background, --setup-auto)
• swamp config command (new command: get, set, list)

These follow the Diátaxis framework used in the manual.