feat: add graph representation of agent network (#15056)

Add a representation of the agent graph. This is now used for:
* Cascade close agents (when I close a parent, it close the kids)
* Cascade resume (oposite)

Later, this will also be used for post-compaction stuffing of the
context

Direct fix for: https://github.com/openai/codex/issues/14458

codex-rs/state/migrations/0021_thread_spawn_edges.sql

@@ -0,0 +1,8 @@
+CREATE TABLE thread_spawn_edges (
+    parent_thread_id TEXT NOT NULL,
+    child_thread_id TEXT NOT NULL PRIMARY KEY,
+    status TEXT NOT NULL
+);
+
+CREATE INDEX idx_thread_spawn_edges_parent_status
+    ON thread_spawn_edges(parent_thread_id, status);