Support compact SessionStart hooks (#21272)

# Why

Compaction replaces the live conversation history, so hooks that use
`SessionStart` to re-inject durable model context need a way to run
again after that rewrite.

Related - #19905 adds dedicated compact lifecycle hooks

# What

- add `compact` as a supported `SessionStart` source and matcher value
- change pending `SessionStart` state from a single slot to a small FIFO
queue so `resume` / `startup` / `clear` can be preserved alongside a
later `compact`
- drain all queued `SessionStart` sources before the next model request,
preserving their original order

# Testing

The new integration coverage verifies both the basic `compact` matcher
path and the stacked `resume` -> `compact` case where both hooks
contribute `additionalContext` to the next model turn.

codex-rs/hooks/schema/generated/session-start.command.input.schema.json

@@ -37,7 +37,8 @@
       "enum": [
         "startup",
         "resume",
-        "clear"
+        "clear",
+        "compact"
       ],
       "type": "string"
     },