docs(conf.py): drop dirhtml from llms.txt builder list, use StrictUndefined

Two follow-up Copilot findings on the curated llms.txt render:

1. Drop `dirhtml` from the builder allow-list. The template hard-codes
   `.html` URLs (`quick-start.html`), which don't exist under `dirhtml`
   output (`quick-start/index.html`). Production publishes via the
   `html`/`readthedocs` builders, so `make dirhtml` would only emit
   a misleading file. Cleaner to skip than to branch URL generation on
   builder type for a path we don't actually ship.

2. Use a Jinja `Environment` with `StrictUndefined` instead of a bare
   `Template`. A typo in `llms.txt.j2` (e.g. `{{ relase }}`) now raises
   at build time instead of silently rendering as an empty string and
   shipping a half-blank `llms.txt`. Also set `keep_trailing_newline`
   so the rendered file's terminating newline is preserved.

Addresses Copilot review feedback on PR #1874.

\xf0\x9f\xa4\x96 Generated by [robots](https://vyos.io)

1 files changed, 12 insertions, 3 deletions

diff --git a/docs/conf.py b/docs/conf.py
index e9b28b71..72c4595a 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -220,15 +220,24 @@ def _prefer_webp(app):
 
 
 def _write_llms_txt(app, exception):
+    # Skip dirhtml: the curated template encodes `.html` URLs (e.g.
+    # `quick-start.html`), which don't exist under `dirhtml` output
+    # (`quick-start/index.html`). Production publishes via the html /
+    # readthedocs builders, so dirhtml output would just be misleading.
     if exception is not None or app.builder.name not in (
-            'html', 'dirhtml', 'readthedocs'):
+            'html', 'readthedocs'):
         return
     from pathlib import Path
-    from jinja2 import Template
+    from jinja2 import Environment, StrictUndefined
     tpl_path = Path(app.srcdir) / '_templates' / 'llms.txt.j2'
     out_path = Path(app.outdir) / 'llms.txt'
     baseurl = (app.config.html_baseurl or '').rstrip('/') + '/'
-    rendered = Template(tpl_path.read_text(encoding='utf-8')).render(
+    # StrictUndefined: missing template variables raise rather than
+    # silently render as empty strings, so a typo in llms.txt.j2 fails
+    # the build instead of shipping a half-blank llms.txt.
+    env = Environment(undefined=StrictUndefined, keep_trailing_newline=True)
+    template = env.from_string(tpl_path.read_text(encoding='utf-8'))
+    rendered = template.render(
         baseurl=baseurl,
         release=app.config.release,
     )