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/shared/src/validators/agent.ts

@@ -59,6 +59,12 @@ export const wakeAgentSchema = z.object({
 
 export type WakeAgent = z.infer<typeof wakeAgentSchema>;
 
+export const resetAgentSessionSchema = z.object({
+  taskKey: z.string().min(1).optional().nullable(),
+});
+
+export type ResetAgentSession = z.infer<typeof resetAgentSessionSchema>;
+
 export const updateAgentPermissionsSchema = z.object({
   canCreateAgents: z.boolean(),
 });