Any mod can add its own screen to Hermes' dock and Apps grid — the same extension point every app we ship (Home, Notes, Journal, Calendar, Reminders, Apps itself) goes through internally.

Registering your app

Declare x-lifestyle-hermes in your own mod's requiredMods, feature-detect the registry, and register once at your script's own load time:

if (window.BMFT && window.BMFT.Hermes && window.BMFT.Hermes.Apps) {
  window.BMFT.Hermes.Apps.register({
    id: "fitness-log",              // required, unique -- duplicates throw
    label: "Fitness Log",           // required
    render: renderFitnessScreen,    // required -- owns screenEl.innerHTML while your app is open
    icon: { svg, tileFrom, tileTo }, // optional -- omit it and you still get a labeled fallback tile, not a blank one
    order: 50,                      // optional -- dock position, ascending; omitted appends after everything else
    pinnable: true,                 // optional -- lets a player favorite/unfavorite your app on and off the dock
    dockExcluded: false,            // optional -- true means Apps-screen-only, never reaches the dock at all, even if pinnable
    dockBadgeCount: () => 3,        // optional -- a live counter badge on your own dock tile
    goBack: (screenEl) => false,    // optional -- handle your own sub-level navigation on the shell's Back button
  });
}

render(screenEl) is called with the shell's screen container every time your dock tile is tapped, and owns that element's innerHTML until the next render. goBack/dockBadgeCount/render are each called inside a try/catch at the call site — a throw degrades gracefully (blank screen, falls through to Home, or no badge) and logs a warning, instead of taking the whole dock down.

A few things worth knowing before you register:

• Ids aren't namespaced. We only reject an exact duplicate id, so pick something specific to your own mod (your-mod-fitness-log, not notes) rather than risk colliding with a future built-in app.
• The favorite cap is shared. Every pinnable: true app — built-in or third-party — competes for the same small pool of dock favorite slots, first-come.
• defaultFavorited only sticks for our own built-in apps. A third-party app can't force itself onto every player's dock unasked; the flag is silently ignored for any id we didn't ship ourselves.
• Built-in apps always sort ahead of third-party ones, on the dock and in the Apps screen's grids alike — your own order only breaks ties among other third-party apps, it can't push you ahead of a built-in app no matter how low you set it.
• dockExcluded is not the same as pinnable + defaultFavorited: false. The latter (Journal's own shape) starts off the dock but the player can still favorite it on later. dockExcluded: true means never, full stop — your app only ever shows in the Apps screen's grid.