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

@@ -0,0 +1,33 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+CREATE TABLE agent_runs (
+    id              TEXT NOT NULL PRIMARY KEY,
+    tenant_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,
+    input_messages  JSONB NOT NULL,
+    result          JSONB,
+    error_message   TEXT,
+    started_at      TIMESTAMPTZ,
+    lease_owner     TEXT,
+    lease_expires_at TIMESTAMPTZ,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_agent_runs_running_lease ON agent_runs (lease_expires_at)
+    WHERE status = 'RUNNING';