New optional plugin (FEATURE_SPACE_INVITER) that detects local homeserver
users who aren't in any Matrix Space and prompts the configured admin via
DM with yes/no/ignore. Persistent prompt log in space_inviter_prompts so
restarts and upgrades don't re-ping for users who already replied.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
