refactor: split oversized hook.ts to respect 200 LOC limit

- Extract types to types.ts
- Extract constants to constants.ts
- Extract session ID helpers to session-id.ts
- Extract recovery logic to recovery.ts

hook.ts reduced from 331 to 164 LOC

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/compaction-context-injector/session-id.ts

@@ -0,0 +1,8 @@
+export function isCompactionAgent(agent: string | undefined): boolean {
+  return agent?.trim().toLowerCase() === "compaction"
+}
+
+export function resolveSessionID(props?: Record<string, unknown>): string | undefined {
+  return (props?.sessionID ??
+    (props?.info as { id?: string } | undefined)?.id) as string | undefined
+}