Add agent config revisions, issue-approval links, and robust migration reconciliation

Add agent_config_revisions table for tracking agent configuration changes with
rollback support. Add issue_approvals junction table linking issues to approvals.
New migrations (0005, 0006) for permissions column and new tables. Rework migration
client with statement-level idempotency checks (table/column/index/constraint
existence) so migrations can be safely retried against partially-migrated databases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/migrations/meta/_journal.json

@@ -36,6 +36,20 @@
       "when": 1771545600000,
       "tag": "0004_issue_identifiers",
       "breakpoints": true
+    },
+    {
+      "idx": 5,
+      "version": "7",
+      "when": 1771545601000,
+      "tag": "0005_chief_luke_cage",
+      "breakpoints": true
+    },
+    {
+      "idx": 6,
+      "version": "7",
+      "when": 1771545602000,
+      "tag": "0006_overjoyed_mister_sinister",
+      "breakpoints": true
     }
   ]
-}
+}