debug config also drives Twig debug + bypasses static cache

When debug is true: enable Twig debug mode (dump(), auto_reload,
DebugExtension), and stop caching sidecar-less pages — Renderer skips
the cache write, the dev router skips serving cache files, and
bootstrap clears stale cache on the first request that reaches PHP.

Also update docs: new Debug mode section on the config page, cache-
opt-out notes (debug + empty index.php sidecar) on the caching page,
and correct the Sass build target to App/css/main.css in AGENTS.md
and the styling page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FZQHKxu24mvojrJDwhWVS

AGENTS.md

@@ -191,7 +191,8 @@ except `public/cache/*` and `novaconium/contact-log.txt`.
   (`php novaconium/bin/<script>.php`) — distinct from
   `bootstrap.php`/`autoload.php`/`config.php`, which are only `require`'d.
 - CSS compiles from `novaconium/sass/main.sass` (indented syntax) to
-  `public/css/main.css`:
-  `sass --load-path=App/sass --load-path=novaconium/sass/defaults --no-source-map novaconium/sass/main.sass public/css/main.css`
+  `App/css/main.css` (project-owned; docker-compose mounts `App/css` to
+  the container's `public/css`, and it's served at `/css/main.css`):
+  `sass --load-path=App/sass --load-path=novaconium/sass/defaults --no-source-map novaconium/sass/main.sass App/css/main.css`
   — commit the regenerated CSS. See `/admin/docs/styling` for a Docker
   fallback if `sass` isn't installed locally.