ci(docs): Fail the docs build on Sphinx warnings

why: The docs job did not pass -W, so a warning printed and the build still
went green. That is not hypothetical: sphinx-autodoc-fastmcp raised
diagnostics through a stdlib logger that Sphinx never saw, so a build could
report a problem and exit 0. Those diagnostics now go through Sphinx's
warning stream, and without -W nothing would act on them.

what: Pass -W --keep-going through the existing sphinxopts variable, so the
justfile is unchanged and a local `just html` keeps its current behaviour.
--keep-going reports every warning in one run rather than stopping at the
first. Verified against this branch's current pin: the build passes, and it
fails when a warning is present, so the gate can actually bite.

Author: tony

.github/workflows/docs.yml

@@ -74,7 +74,7 @@ jobs:
       - name: Build documentation
         if: env.PUBLISH == 'true'
         run: |
-          cd docs && just html
+          cd docs && just sphinxopts="-W --keep-going" html
 
       - name: Configure AWS Credentials
         if: env.PUBLISH == 'true'