docs(developer-guide): document htui/hgui worktree UI dev helpers (#64783)

Add a developer-guide page for running the Ink TUI and Electron desktop
app from a git worktree without a full npm install per checkout, via the
htui/hgui shell helpers that share node_modules from a canonical deps
checkout by symlink (falling back to a local npm ci when the lockfile
diverges). Registers it in the sidebar, cross-links from the TUI and git
-worktrees pages, and documents the previously-undocumented
HERMES_DESKTOP_PYTHON / HERMES_DESKTOP_DEV_SERVER env vars the desktop
backend reads.

website/docs/reference/environment-variables.md

@@ -510,6 +510,8 @@ Three dashboard-auth providers ship in the box. For a remote Hermes Desktop conn
 | `HERMES_DESKTOP_HERMES_ROOT` | Desktop source-checkout override used by `hermes desktop --hermes-root`; checked before the packaged first-launch install or an existing `hermes` on `PATH`. |
 | `HERMES_DESKTOP_IGNORE_EXISTING` | Set to `1` to make Desktop ignore an existing `hermes` on `PATH` during backend resolution. Equivalent to `hermes desktop --ignore-existing`. |
 | `HERMES_DESKTOP_CWD` | Initial project directory for Desktop chat sessions. Set by `hermes desktop --cwd`. |
+| `HERMES_DESKTOP_PYTHON` | Absolute path to a Python interpreter for the backend, checked before Electron auto-resolves one for the source checkout. Used by worktree dev helpers (see [TUI & Desktop from Worktrees](../developer-guide/worktree-ui-dev.md)) to reuse a shared venv. |
+| `HERMES_DESKTOP_DEV_SERVER` | Vite dev-server URL the Electron shell loads instead of the packaged bundle (e.g. `http://127.0.0.1:5174`). Set automatically by `npm run dev`; only relevant when hacking on the app. |
 
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