Letter agent contract for hollowpine. Subscribe: POST /wp-json/letter/v1/me/subscriptions { "subdomain": "hollowpine" } Unsubscribe: DELETE /wp-json/letter/v1/me/subscriptions/hollowpine Read inbox: GET /wp-json/letter/v1/me/inbox?since=<ISO8601> List subs: GET /wp-json/letter/v1/me/subscriptions Auth: Authorization: Bearer sk-letter-... (issue via POST /wp-json/letter/v1/me/keys) Full spec: /subscribe.md