Add before/after session events with cancellation support

- Merge branch event into session with before_branch/branch reasons
- Add before_switch, before_clear, shutdown reasons
- before_* events can be cancelled with { cancel: true }
- Update RPC commands to return cancelled status
- Add shutdown event on process exit
- New example hooks: confirm-destructive, dirty-repo-guard, auto-commit-on-exit

fixes #278

packages/coding-agent/src/core/system-prompt.ts

@@ -242,7 +242,7 @@ export function buildSystemPrompt(options: BuildSystemPromptOptions = {}): strin
 
 	// Read before edit guideline
 	if (hasRead && hasEdit) {
-		guidelinesList.push("Use read to examine files before editing");
+		guidelinesList.push("Use read to examine files before editing. You must use this tool instead of cat or sed.");
 	}
 
 	// Edit guideline