Fix restart-required infinite loop from stale retained MQTT discovery

[ezpl]

Fixes #63

Three fixes to break the restart-required infinite loop:

• config_flow.py: delete stale issue from old device name before creating a new one on rename; also delete the issue on name-match discovery (safety net for lingering issues)
• repairs.py: dismiss the issue immediately when user clicks "Restart" in the fix flow, before calling homeassistant.restart
• init.py: sweep all leftover hass_agent issues on startup so renamed devices that never got dismissed don't pile up forever