Add background update checks

[Staphylococcus]

Summary

• add lg-buddy updates background-check as the service-owned path for scheduled update checks
• add updates.auto_check and updates.channel settings backed by config.env
• install a user systemd timer/service for background checks and include them in release bundles
• preserve update settings across configure/install reruns and document the new behavior

Part of #27.

Validation

• cargo fmt --all
• cargo test -p lg-buddy
• cargo clippy -p lg-buddy --all-targets --all-features -- -D warnings
• bash -n install.sh uninstall.sh configure.sh bin/LG_Buddy_Common scripts/build-release-bundle.sh scripts/test-release-bundle.sh scripts/publish-release-assets.sh
• git diff --check
• release bundle smoke test with scripts/test-release-bundle.sh --skip-pip-install

[Copilot]

Pull request overview

This PR adds a scheduled, user-level update-check mechanism for release-bundle installs by introducing a service-owned updates background-check command, new update-related settings persisted in config.env, and systemd user units to run checks automatically.

Changes:

• Add lg-buddy updates background-check and wire it to LG_Buddy_update_check.service + .timer.
• Introduce updates.auto_check and updates.channel settings (backed by config.env) and hook updates.auto_check to enable/disable the user timer via the settings apply layer.
• Update installer/uninstaller, release bundle scripts, and documentation/tests to include and validate the new behavior.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
uninstall.sh	Removes/disables the new user update-check timer/service and drop-in override on uninstall.
systemd/LG_Buddy_update_check.timer	Adds a user timer to schedule background update checks.
systemd/LG_Buddy_update_check.service	Adds a oneshot user service invoking lg-buddy updates background-check.
scripts/test-release-bundle.sh	Extends bundle/install smoke tests to assert update units and new settings behavior.
scripts/build-release-bundle.sh	Includes the new update-check systemd units in the release bundle.
README.md	Documents background update checks and new settings keys.
install.sh	Installs update-check user units, persists config override, and enables/disables timer based on config.
docs/user-guide.md	Documents the new updates background-check surface and update settings.
docs/runtime-event-handler-map.md	Adds the user update-check timer path to the runtime handler map.
docs/defaults-and-configuration.md	Documents defaults and opt-out model for background update checks and channel selection.
docs/architecture-overview.md	Updates architecture docs to include scheduled update checks and the new command surface.
crates/lg-buddy/tests/features/settings.feature	Adds BDD coverage for update settings visibility and persistence.
crates/lg-buddy/src/updates.rs	Implements updates background-check, reads update settings, and reuses existing update-check + notification logic.
crates/lg-buddy/src/settings.rs	Adds update settings definitions and implements a systemd apply strategy to enable/disable the timer.
crates/lg-buddy/src/lib.rs	Updates CLI help and parsing tests for the new updates background-check command.
configure.sh	Preserves/writes update-check settings into config.env across configure reruns.
bin/LG_Buddy_Common	Adds defaults + validation/loading for new update config keys.

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