fix(update): reload LaunchAgent after app activation - #313

What changed

• Define LaunchAgent re-registration as part of every actual PV app activation.
• Keep the validated PV-owned plist path from update preflight.
• After activating a new release, perform tolerant bootout, bootstrap, then kickstart.
• Repeat the same reload sequence after restoring the previous release during rollback.
• Preserve app-current behavior: stale owned plist content may be normalized, but no daemon reload occurs.
• Cover the launchd “already unloaded” response while still requiring bootstrap and kickstart.

Why

A current plist previously produced NotRequired, so PV only kickstarted the existing launchd registration after switching ~/.pv/bin/pv. launchd could therefore continue using the executable resolved from the previous registration. Rollback had the same issue.

Reloading the validated owned plist after every actual binary switch makes launchd resolve the active symlink again. The existing tolerant bootout handling preserves the already-unloaded case.

Review commits

1. docs(design): require LaunchAgent reload after activation
2. fix(cli): reload LaunchAgent after app activation
3. test(cli): cover unloaded LaunchAgent update

Checks

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo nextest run -p cli --test update (38 passed)

Closes #300