cleanup: remove rekey flow, fall through to re-registration instead
The rekey flow only handled a corrupted state (garden exists but OAuth
client doesn't) that isn't a normal operational scenario. Simplify the
client-side fallback: when reauthentication fails, clear credentials
and re-register as a new garden.
Removes: rekey API endpoint, GardenRekey schema, rekey_garden/2,
Registration.rekey/3, and all client-side rekey logic.
sow-157
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>