Distinguish forked sessions in session-start hooks (#44349)

## Why

Forked threads reported `startup`, causing startup hooks to run again even when their context was inherited from the parent. Resuming with supplied history also reported `startup` instead of `resume`.

## What changed

- Add `fork` as a `SessionStart` source and expose it in the hook input schema.
- Report `fork` for histories with a fork parent and `resume` for supplied histories without one.
- Continue dispatching `SubagentStart` for spawned subagents with either fresh or forked context.

## Testing

Add regression coverage for fork hook matching without duplicate startup context, resume hooks with supplied history, and `SubagentStart` context injection for fresh and forked subagents.

GitOrigin-RevId: 5ac1ae4ab8adcbbbceb8cb0ffb9b32b6402c585b

codex-rs/core/src/session/session.rs

@@ -1682,10 +1682,15 @@ impl Session {
             sess.schedule_startup_prewarm(sess.get_prompt_base_instructions().await.text)
                 .await;
             let session_start_source = match &initial_history {
-                InitialHistory::Resumed(_) => codex_hooks::SessionStartSource::Resume,
-                InitialHistory::New | InitialHistory::Forked(_) => {
-                    codex_hooks::SessionStartSource::Startup
+                InitialHistory::Forked(_) if forked_from_id.is_some() => {
+                    codex_hooks::SessionStartSource::Fork
                 }
+                // `thread/resume` with supplied history uses `Forked` internally
+                // without a fork parent, so it should still report `resume`.
+                InitialHistory::Resumed(_) | InitialHistory::Forked(_) => {
+                    codex_hooks::SessionStartSource::Resume
+                }
+                InitialHistory::New => codex_hooks::SessionStartSource::Startup,
                 InitialHistory::Cleared => codex_hooks::SessionStartSource::Clear,
             };