Add agent task sessions table, session types, and programmatic DB backup

Add agent_task_sessions table for per-task session state keyed by
(agent, adapter, taskKey). Add AgentTaskSession type, resetAgentSessionSchema
validator, and sessionDisplayId/sessionParamsJson to AgentRuntimeState.

Rework migration hash-resolution fallback ordering to prefer hash-based
matching over timestamp-based journal matching. Move backup-db.sh logic
into packages/db/src/backup.ts for programmatic use and simplify the shell
script to call the TypeScript implementation.

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

packages/db/src/schema/index.ts

@@ -3,6 +3,7 @@ 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 { agentTaskSessions } from "./agent_task_sessions.js";
 export { agentWakeupRequests } from "./agent_wakeup_requests.js";
 export { projects } from "./projects.js";
 export { goals } from "./goals.js";