Scope PGCheckpointer queries by tenant GID

Each Save and Load now derives tenant_id from the run GID and pins it
in the WHERE clause. A caller that supplies an ID from another tenant
fails closed instead of silently reading or overwriting cross-tenant
checkpoint data. Also rejects oversize checkpoints on load as a
read-side guard against a tampered or migrated row exceeding
MaxCheckpointBytes.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/coredata/migrations/20260424T173529Z.sql

@@ -15,7 +15,7 @@
 CREATE TABLE agent_runs (
     id              TEXT NOT NULL PRIMARY KEY,
     tenant_id       TEXT NOT NULL,
-    organization_id TEXT NOT NULL,
+    organization_id TEXT NOT NULL REFERENCES organizations(id) ON DELETE CASCADE,
     start_agent_name TEXT NOT NULL,
     status          TEXT NOT NULL DEFAULT 'PENDING',
     checkpoint      JSONB,
@@ -29,8 +29,5 @@ CREATE TABLE agent_runs (
     updated_at      TIMESTAMPTZ NOT NULL DEFAULT now()
 );
 
-CREATE INDEX idx_agent_runs_status ON agent_runs (status)
-    WHERE status IN ('PENDING', 'RUNNING', 'SUSPENDED');
-CREATE INDEX idx_agent_runs_organization_status ON agent_runs (organization_id, status, created_at);
 CREATE INDEX idx_agent_runs_running_lease ON agent_runs (lease_expires_at)
     WHERE status = 'RUNNING';