WIP: Add theming system with /theme command

- Consolidated theme system into single src/theme/ directory
- Created Theme class with fg(), bg(), bold(), italic(), underline()
- Added dark and light built-in themes with 36 color tokens
- Support for custom themes in ~/.pi/agent/themes/*.json
- JSON schema for theme validation
- Theme selector UI with /theme command
- Save theme preference to settings
- Uses chalk for text formatting to preserve colors

TODO:
- Replace hardcoded colors throughout TUI components
- Apply markdown theming to Markdown components
- Add theme support to all TUI elements

packages/coding-agent/src/settings-manager.ts

@@ -7,6 +7,7 @@ export interface Settings {
 	defaultProvider?: string;
 	defaultModel?: string;
 	queueMode?: "all" | "one-at-a-time";
+	theme?: string;
 }
 
 export class SettingsManager {
@@ -88,4 +89,13 @@ export class SettingsManager {
 		this.settings.queueMode = mode;
 		this.save();
 	}
+
+	getTheme(): string | undefined {
+		return this.settings.theme;
+	}
+
+	setTheme(theme: string): void {
+		this.settings.theme = theme;
+		this.save();
+	}
 }