feat(coding-agent): add compaction reason and willRetry to extension compact events (#5962)

session_before_compact and session_compact now carry
reason ("manual" | "threshold" | "overflow") and willRetry, matching
what the RPC protocol already exposes via auto_compaction_start/end.
Extensions can now distinguish manual /compact from threshold
auto-compaction and overflow recovery through the public API instead
of monkeypatching AgentSession internals.

packages/coding-agent/docs/compaction.md

@@ -276,7 +276,7 @@ Fired before auto-compaction or `/compact`. Can cancel or provide custom summary
 
 ```typescript
 pi.on("session_before_compact", async (event, ctx) => {
-  const { preparation, branchEntries, customInstructions, signal } = event;
+  const { preparation, branchEntries, customInstructions, reason, willRetry, signal } = event;
 
   // preparation.messagesToSummarize - messages to summarize
   // preparation.turnPrefixMessages - split turn prefix (if isSplitTurn)
@@ -287,6 +287,8 @@ pi.on("session_before_compact", async (event, ctx) => {
   // preparation.settings - compaction settings
 
   // branchEntries - all entries on current branch (for custom state)
+  // reason - "manual" (/compact), "threshold", or "overflow"
+  // willRetry - whether the aborted turn is retried after compaction (overflow recovery)
   // signal - AbortSignal (pass to LLM calls)
 
   // Cancel: