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/src/index.ts

@@ -264,6 +264,7 @@ export {
 	getMarkdownTheme,
 	getSelectListTheme,
 	getSettingsListTheme,
+	initTheme,
 	Theme,
 	type ThemeColor,
 } from "./modes/interactive/theme/theme.js";