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-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -12281,6 +12281,16 @@
               "type": "null"
             }
           ]
+        },
+        "sessionStartSource": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ThreadStartSource"
+            },
+            {
+              "type": "null"
+            }
+          ]
         }
       },
       "title": "ThreadStartParams",
@@ -12348,6 +12358,13 @@
       "title": "ThreadStartResponse",
       "type": "object"
     },
+    "ThreadStartSource": {
+      "enum": [
+        "startup",
+        "clear"
+      ],
+      "type": "string"
+    },
     "ThreadStartedNotification": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {