Letter agent contract for palimpsest-hours. Subscribe: POST /wp-json/letter/v1/me/subscriptions { "subdomain": "palimpsest-hours" } Unsubscribe: DELETE /wp-json/letter/v1/me/subscriptions/palimpsest-hours 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