rename: implement advertised backward-compat + prune dead community plugins

Address gaps where PR #537's "Backward compatibility" section was advertised
but not implemented, and clean up the community plugin list.

Env vars (FEEDBACK_* canonical, legacy SLOPSMITH_* honoured):
- New lib/env_compat.py (getenv_compat / env_flag_compat) + tests. server.py
  (_env_flag + all FEEDBACK_* reads), diagnostics_hardware, gp2midi and
  tailwind_rebuild now resolve the legacy alias, so existing SLOPSMITH_UI /
  SLOPSMITH_PLUGINS_DIR / etc. deployments keep working.
- Fix the rename collapsing plugins/__init__.py and minigames/routes.py from
  `FEEDBACK_PLUGINS_DIR or SLOPSMITH_PLUGINS_DIR` into a redundant
  `FEEDBACK_ or FEEDBACK_` (the fallback was silently lost).

Storage (app.js update-channel):
- Read feedBack-update-channel, fall back to legacy slopsmith-update-channel,
  and clear the legacy key on write — so a user's update-channel preference
  survives the rename instead of resetting to "stable".

Community plugin list (README): the rename rewrote third-party repo URLs we
don't own. Probed every one; their owners never renamed, so:
- Restore the 13 live community plugins to their real slopsmith-* names.
- Prune 6 that are 404 to the public (topkoa splitscreen/stems, OmikronApex
  tuner, Jafz2001 nam-rig-builder, DeathlySin song-preview, Erikcb91 shuffle).
- Fix a pre-existing Guitar Theory clone-command typo (nam-tone -> guitar-theory).

Verified: env_compat 7/7, JS 819/819, pytest 1690 collected / 0 import errors,
rename-sensitive + startup suites green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/minigames/routes.py

@@ -336,7 +336,7 @@ def setup(app, context):
     # Duplicates are removed via a seen-set keyed on resolved paths.
     def _resolve_plugin_dirs():
         roots = []
-        env_dir = os.environ.get("FEEDBACK_PLUGINS_DIR") or os.environ.get("FEEDBACK_PLUGINS_DIR")
+        env_dir = os.environ.get("FEEDBACK_PLUGINS_DIR") or os.environ.get("SLOPSMITH_PLUGINS_DIR")
         if env_dir:
             roots.append(Path(env_dir))
         # Built-in plugins/ next to server.py (one level above this file's