Refactor SessionEventBase to pass sessionManager and modelRegistry

Breaking changes to hook types:
- SessionEventBase now passes sessionManager and modelRegistry directly
- before_compact: passes preparation, previousCompactions (newest first)
- before_switch: has targetSessionFile; switch: has previousSessionFile
- Removed resolveApiKey (use modelRegistry.getApiKey())
- getSessionFile() returns string | undefined for in-memory sessions

Updated:
- All session event emissions in agent-session.ts
- Hook examples (custom-compaction.ts, auto-commit-on-exit.ts, confirm-destructive.ts)
- Tests (compaction-hooks.test.ts, compaction-hooks-example.test.ts)
- export-html.ts guards for in-memory sessions

packages/coding-agent/src/core/export-html.ts

@@ -1343,6 +1343,9 @@ export function exportSessionToHtml(
 	const opts: ExportOptions = typeof options === "string" ? { outputPath: options } : options || {};
 
 	const sessionFile = sessionManager.getSessionFile();
+	if (!sessionFile) {
+		throw new Error("Cannot export in-memory session to HTML");
+	}
 	const content = readFileSync(sessionFile, "utf8");
 	const data = parseSessionFile(content);