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/claude-rules.ts

@@ -61,7 +61,7 @@ export default function claudeRulesExtension(pi: ExtensionAPI) {
 	});
 
 	// Append available rules to system prompt
-	pi.on("before_agent_start", async () => {
+	pi.on("before_agent_start", async (event) => {
 		if (ruleFiles.length === 0) {
 			return;
 		}
@@ -69,7 +69,10 @@ export default function claudeRulesExtension(pi: ExtensionAPI) {
 		const rulesList = ruleFiles.map((f) => `- .claude/rules/${f}`).join("\n");
 
 		return {
-			systemPromptAppend: `
+			systemPrompt:
+				event.systemPrompt +
+				`
+
 ## Project Rules
 
 The following project rules are available in .claude/rules/:

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

@@ -1,8 +1,8 @@
 /**
  * Pirate Extension
  *
- * Demonstrates using systemPromptAppend in before_agent_start to dynamically
- * modify the system prompt based on extension state.
+ * Demonstrates modifying the system prompt in before_agent_start to dynamically
+ * change agent behavior based on extension state.
  *
  * Usage:
  * 1. Copy this file to ~/.pi/agent/extensions/ or your project's .pi/extensions/
@@ -25,10 +25,13 @@ export default function pirateExtension(pi: ExtensionAPI) {
 	});
 
 	// Append to system prompt when pirate mode is enabled
-	pi.on("before_agent_start", async () => {
+	pi.on("before_agent_start", async (event) => {
 		if (pirateMode) {
 			return {
-				systemPromptAppend: `
+				systemPrompt:
+					event.systemPrompt +
+					`
+
 IMPORTANT: You are now in PIRATE MODE. You must:
 - Speak like a stereotypical pirate in all responses
 - Use phrases like "Arrr!", "Ahoy!", "Shiver me timbers!", "Avast!", "Ye scurvy dog!"

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}`,
 			};
 		}
 	});

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

@@ -191,4 +191,16 @@ export default function (pi: ExtensionAPI) {
 			ctx.ui.notify(`SSH mode: ${ssh.remote}:${ssh.remoteCwd}`, "info");
 		}
 	});
+
+	// Replace local cwd with remote cwd in system prompt
+	pi.on("before_agent_start", async (event) => {
+		const ssh = getSsh();
+		if (ssh) {
+			const modified = event.systemPrompt.replace(
+				`Current working directory: ${localCwd}`,
+				`Current working directory: ${ssh.remoteCwd} (via SSH: ${ssh.remote})`,
+			);
+			return { systemPrompt: modified };
+		}
+	});
 }