feat(FN-1253): implement task checkout leasing end-to-end

- Add checkout lease types and conflict error exports, plus DB schema v20 migration for checkedOutBy/checkedOutAt
- Persist checkout lease fields in TaskStore and add AgentStore checkout/release/force-release/get-holder operations
- Add dashboard checkout API routes for acquire/release/force-release/status with explicit 409 conflict and 403 holder enforcement
- Enforce checkout ownership in heartbeat execution with graceful checkout_conflict exits when another agent holds the lease
- Expand core and dashboard test coverage for schema, store behavior, API routes, and leasing workflows, and document leasing behavior in AGENTS.md

packages/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 19;
+const SCHEMA_VERSION = 20;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -742,6 +742,13 @@ export class Database {
         this.db.exec("CREATE INDEX IF NOT EXISTS idxAiSessionsLock ON ai_sessions(lockedByTab)");
       });
     }
+
+    if (version < 20) {
+      this.applyMigration(20, () => {
+        this.addColumnIfMissing("tasks", "checkedOutBy", "TEXT");
+        this.addColumnIfMissing("tasks", "checkedOutAt", "TEXT");
+      });
+    }
   }
 
   /**