Bootstrap the per-user Python venv from the wrapper

The Project Manager GUI does not set up O3DE's Python venv itself; it only
errors ("Missing python venv file .../pyvenv.cfg") when it's absent, which
cascades into "Failed to get engine info" / "Failed to initialize". Nothing
was running get_python.sh on launch.

The wrapper now runs python/get_python.sh when python.sh reports the venv is
missing/stale. get_python.sh downloads Python, creates the venv under the
writable ~/.o3de, and installs the o3de CLI from the prebuilt sdist. It is
idempotent and self-heals a half-built venv (venv creation uses --clear).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -127,10 +127,13 @@ These were confirmed by inspecting the v26.05 package (`opt/O3DE/26.05/…`):
   engine compiles project code at runtime. Those live in the SDK. Users therefore
   pull the SDK runtime (larger than Platform) on install — Flatpak does this
   automatically from Flathub.
-- **Python is per-user, not baked in.** On first launch O3DE downloads its Python
-  runtime and builds a venv under `~/.o3de` (writable), pip-installing its deps
-  there. The build does **not** bundle Python — the venv is keyed to the user's
-  home path, so it can only be created at runtime.
+- **Python is per-user, not baked in.** O3DE needs a Python venv under `~/.o3de`
+  (writable), keyed to the engine path, so it can only be created at runtime — the
+  build can't bundle it. The Project Manager GUI does **not** create it itself, so
+  the wrapper (`o3de-wrapper.sh`) runs `python/get_python.sh` on launch when the
+  venv is missing or stale. The **first launch downloads Python and can take
+  several minutes with no window on screen** — that's expected; subsequent launches
+  are instant. (cmake, which `get_python.sh` needs, comes from the Sdk runtime.)
 - **The o3de CLI install (read-only `/app`).** On first launch O3DE installs its
   own `o3de` CLI from `scripts/o3de` into the per-user `~/.o3de` venv. By default
   `python/get_python.sh` does an *editable* install (`pip install -e`), which