fix(db): prevent migration-test database drop + correct AEG-X-004 evidence

Tests now guard against accidental drop of kartsell_migration_test by throwing
when the credential source DB is the destructive rehearsal target. Distinct
credential DB (kartselldb_test) prevents config collision.

AEG-X-004 evidence consolidated: rehearsal .trx files + preflight markdown
documented. Schema 0032 (shadow_run_queued_status_contract) verified
fresh/upgrade/recovery on isolated DB.

AGENTS.md: Necessity-driven (guard against destructive accident); no new feature.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

evidence/AEG-X-004/production-readonly-preflight-20260806.md

@@ -0,0 +1,27 @@
+# AEG-X-004 Production Read-only Preflight — 2026-08-06
+
+## Source / Assumption / Unknown / Decision Required
+
+- Source: `publish/appsettings.json` connection string, read-only Npgsql query through the configured local PostgreSQL connection.
+- Assumption: `kartselldb` is the intended production database because it is the database named by the published application configuration.
+- Unknown: none for the `0032` journal/constraint check; a separate release receipt still needs to be attached.
+- Decision Required: DBA/Release owner must approve the normal DbUp deployment and preserve its receipt; no direct journal edit or migration execution was performed.
+
+## Observed result
+
+```text
+Database: kartselldb
+User: kartsell
+DbUp journal table: public.kartsell_schema_versions
+0032 journal row present: True
+Legacy __dbup_schema_history table present: True (not used by the current DbMigrator)
+shadow_run.check_status constraint includes Queued: True
+```
+
+## Gate decision
+
+`PHASE-1-SHADOW-RUN` remains `BLOCKED` pending the deployment receipt and VersionSet approval. The active DbUp journal and constraint are compatible with the application. No migration or enqueue command was issued.
+
+## Safe next action
+
+DBA/Release owner must attach the deployment receipt, then approve the VersionSet freeze and Shadow-only enqueue. Direct SQL journal edits and manual Shadow enqueue remain prohibited.