Add AgentRun entity and PG checkpoint store

Create agent_runs table with lease-based concurrency control.
AgentRun entity follows standard coredata patterns with Scoper,
StrictNamedArgs, and cursor pagination. PGCheckpointStore implements
agent.CheckpointStore backed by the checkpoint JSONB column with
version validation and 10 MiB size guard. Register AgentRunEntityType
as entity type 75.

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

pkg/coredata/migrations/20260424T120000Z.sql

@@ -0,0 +1,37 @@
+-- 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,
+    start_agent_name TEXT NOT NULL,
+    status          TEXT NOT NULL DEFAULT 'PENDING',
+    checkpoint      JSONB,
+    input_messages  JSONB NOT NULL,
+    result          JSONB,
+    error_message   TEXT,
+    stop_requested  BOOLEAN NOT NULL DEFAULT FALSE,
+    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_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';