docs(adr): fill the ADR-0003 ↔ PR backlink (factory#19)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vibe/ADR/0003-sandbox-state-lifecycle.md

@@ -92,4 +92,4 @@ After rehearsing in the sandbox, the change is captured as a reviewable diff usi
 - `factory` `postgres/iac/main.tf` — the per-instance flatten that owns each database by its `<app>_role` / `<app>_<env>_role`; `erp-sandbox` is owned by `erp_sandbox_role`, prod `erp` by `erp_role`, which is why the sandbox cannot drop prod.
 - `tools` `hashicorp-vault/iac/modules/app_roles/main.tf` — the dynamic-credential role whose creation statement grants only `GRANT <app>_role TO {{name}}` (membership only), so `postgres/creds/erp-sandbox` carries no rights on the prod database.
 - `erp` `.claude/skills/dolibarr-data-snapshot/` — the read-only, content-addressable snapshot skill used to capture the reviewable before/after diff at promote time and to verify the reset round-trip.
-- PRs: this ADR is introduced by PR factory#NN (links back to this file). <!-- TODO: fill PR number on open -->
+- PRs: this ADR is introduced by [PR factory#19](https://gitea.arcodange.lab/arcodange-org/factory/pulls/19) (links back to this file).