fix(theme-cycler): persist theme selection to settings.json

[malammar]

Problem

theme-cycler.ts calls ctx.ui.setTheme() to switch themes at runtime but never writes the selection back to settings.json. This means any theme switched via /theme, Ctrl+X, or Ctrl+Q is lost on restart — pi always reverts to whatever "theme" was last set in settings.json.

Fix

Added a persistTheme() helper that writes the selected theme name to ~/.pi/agent/settings.json after every ctx.ui.setTheme() call. This mirrors how pi's own /settings command persists the theme.

function persistTheme(themeName: string): void {
    try {
        const settingsPath = path.join(os.homedir(), ".pi", "agent", "settings.json");
        const raw = fs.existsSync(settingsPath) ? fs.readFileSync(settingsPath, "utf-8") : "{}";
        const settings = JSON.parse(raw);
        settings.theme = themeName;
        fs.writeFileSync(settingsPath, JSON.stringify(settings, null, 2) + "\n", "utf-8");
    } catch {
        // best-effort
    }
}

Called after all 3 occurrences of ctx.ui.setTheme() in the file:

• cycleTheme() — Ctrl+X / Ctrl+Q
• /theme <name> direct set
• /theme select picker

Testing

1. Launch pi with -e extensions/theme-cycler.ts
2. Run /theme and select any theme
3. Restart pi — theme is now preserved