Support clear SessionStart source (#17073)

## Motivation

The `SessionStart` hook already receives `startup` and `resume` sources,
but sessions created from `/clear` previously looked like normal startup
sessions. This makes it impossible for hook authors to distinguish
between these with the matcher.

## Summary

- Add `InitialHistory::Cleared` so `/clear`-created sessions can be
distinguished from ordinary startup sessions.
- Add `SessionStartSource::Clear` and wire it through core, app-server
thread start params, and TUI clear-session flow.
- Update app-server protocol schemas, generated TypeScript, docs, and
related tests.


https://github.com/user-attachments/assets/9cae3cb4-41c7-4d06-b34f-966252442e5c

codex-rs/app-server/tests/suite/v2/skills_list.rs

@@ -241,6 +241,7 @@ async fn skills_changed_notification_is_emitted_after_skill_change() -> Result<(
             developer_instructions: None,
             personality: None,
             ephemeral: None,
+            session_start_source: None,
             dynamic_tools: None,
             mock_experimental_field: None,
             experimental_raw_events: false,