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/schema/index.ts

@@ -1,11 +1,13 @@
 export { companies } from "./companies.js";
 export { agents } from "./agents.js";
+export { agentConfigRevisions } from "./agent_config_revisions.js";
 export { agentApiKeys } from "./agent_api_keys.js";
 export { agentRuntimeState } from "./agent_runtime_state.js";
 export { agentWakeupRequests } from "./agent_wakeup_requests.js";
 export { projects } from "./projects.js";
 export { goals } from "./goals.js";
 export { issues } from "./issues.js";
+export { issueApprovals } from "./issue_approvals.js";
 export { issueComments } from "./issue_comments.js";
 export { heartbeatRuns } from "./heartbeat_runs.js";
 export { heartbeatRunEvents } from "./heartbeat_run_events.js";