Allow extensions to modify system prompt in before_agent_start

- Add systemPrompt to BeforeAgentStartEvent so extensions can see current prompt
- Change systemPromptAppend to systemPrompt in BeforeAgentStartEventResult for full replacement
- Extensions can now chain modifications (each sees the result of previous)
- Update ssh.ts to replace local cwd with remote cwd in system prompt
- Update pirate.ts, claude-rules.ts, preset.ts to use new API

fixes #575

packages/coding-agent/examples/extensions/preset.ts

@@ -345,10 +345,10 @@ export default function presetExtension(pi: ExtensionAPI) {
 	});
 
 	// Inject preset instructions into system prompt
-	pi.on("before_agent_start", async () => {
+	pi.on("before_agent_start", async (event) => {
 		if (activePreset?.instructions) {
 			return {
-				systemPromptAppend: activePreset.instructions,
+				systemPrompt: `${event.systemPrompt}\n\n${activePreset.instructions}`,
 			};
 		}
 	});