feat(herdr): add automatic no-sleep plugin

[princejoogie]

Summary

• add an installable Herdr plugin that keeps macOS awake while any live agent exists
• aggregate agents across named Herdr sessions and retain the 20% battery cutoff
• manage caffeinate, lid-aware brightness, plugin enable/disable, updates, and privileged cleanup
• install a narrowly scoped sudoers rule for only the two required pmset commands
• document installation/removal and add a minor changeset

Verification

• bash -n herdr-plugin/*.sh
• shellcheck herdr-plugin/*.sh
• git diff --check
• Herdr manifest linked and validated on Herdr 0.8.0
• live local installation verified with active agents
• plugin disable restored SleepDisabled=0 and stopped helper processes
• plugin re-enable restored SleepDisabled=1, caffeinate, and the lid watcher automatically