Drop agent-run lease columns in one migration

The intermediate migration added lease_generation only for the next
migration to remove it. Fold the drops into 20260608T090000Z with IF
EXISTS so fresh installs and databases that already ran 20260607 both
migrate cleanly.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/coredata/migrations/20260607T060000Z.sql

@@ -1,17 +0,0 @@
--- 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.
-
-ALTER TABLE agent_runs
-	ADD COLUMN lease_generation BIGINT NOT NULL DEFAULT 0,
-	DROP COLUMN lease_owner;

pkg/coredata/migrations/20260608T090000Z.sql

@@ -21,5 +21,6 @@
 DROP INDEX IF EXISTS idx_agent_runs_running_lease;
 
 ALTER TABLE agent_runs
-	DROP COLUMN lease_expires_at,
-	DROP COLUMN lease_generation;
+	DROP COLUMN IF EXISTS lease_owner,
+	DROP COLUMN IF EXISTS lease_expires_at,
+	DROP COLUMN IF EXISTS lease_generation;