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/migrations/meta/_journal.json

@@ -50,6 +50,13 @@
       "when": 1771545602000,
       "tag": "0006_overjoyed_mister_sinister",
       "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "7",
+      "when": 1771545603000,
+      "tag": "0007_new_quentin_quire",
+      "breakpoints": true
     }
   ]
 }