migrations: 0028 — backfill ON DELETE clauses on legacy FKs

Retrofits the ON DELETE pattern learned after v1 shipped (CASCADE for
tenant-scoped children, SET NULL for historical references, RESTRICT where
the domain type is non-Option) onto tables from 0001-0006 and 0026. Two
deliberate exceptions kept as NO ACTION: audit_log.workspace_id (audit is
append-only and must outlive workspace deletes) and thread_messages.from_agent
(history stays attributable via agents.deleted_at). Two stay NOT NULL as
RESTRICT (agents.managed_by, installed_skills.installed_by) because the
Rust domain type is UserId, not Option<UserId>.

Also includes the previously-orphan .sqlx cache for the usage_events query
in cm-runtime/tests/run_loop.rs, which needed re-recording after the FK
changes touched the metadata.

.sqlx/query-47c1cee8591250819d22e87058c6011bebb77eaf93c1cfa2535db42221d8ecf3.json

@@ -0,0 +1,34 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT tokens_in, tokens_out, credits FROM usage_events\n               WHERE workspace_id = $1",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "tokens_in",
+        "type_info": "Int8"
+      },
+      {
+        "ordinal": 1,
+        "name": "tokens_out",
+        "type_info": "Int8"
+      },
+      {
+        "ordinal": 2,
+        "name": "credits",
+        "type_info": "Numeric"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Uuid"
+      ]
+    },
+    "nullable": [
+      false,
+      false,
+      false
+    ]
+  },
+  "hash": "47c1cee8591250819d22e87058c6011bebb77eaf93c1cfa2535db42221d8ecf3"
+}