docs: join the Python build to the libtmux.org shared shell

[tony]

Summary

• Add docs/_static/libtmux-org.css, the design-token adapter mapping the shared --lt-* tokens onto Furo's own --color-* contract. tokens.css restyles nothing by itself; without this translation table a Sphinx page carries the variables unused in the cascade and still paints Furo's stock blue.
• Load the shell's header, footer, and version switcher through /_shell/shell.js, so the Python build wears the same chrome as the other ports.
• Redirect Sphinx's built-in search page to the shell's Pagefind search at /search/. Furo's search UI needs a client-side index this build does not emit at that path.
• Pin sphinx-gp-llms to a fork carrying the fix for .md twin links on genindex and py-modindex, which have no twin — four dead links in the assembled site.

Blocked on the sphinx-gp-llms pin

[tool.uv.sources] resolves sphinx-gp-llms from ssh://git@github.com/tony/gp-sphinx.git, which is private. uv sync --all-extras --dev is the install step in both tests.yml and docs.yml, and it fails for anyone without a key on that repo:

git@github.com: Permission denied (publickey).

Every build leg of the tmux matrix fails at that install step, before a test runs.

The pin lives on this branch because libtmux.org's assembly reads docs-site directly; the package published to PyPI is unaffected. Merging to master needs the fix released or landed on the public gp-sphinx, and the [tool.uv.sources] and [tool.uv] override-dependencies blocks then dropped.

Changes

docs/conf.py: html_css_files gains libtmux-org.css, listed after css/custom.css so its overrides win the cascade at equal specificity; html_js_files gains a deferred /_shell/shell.js; templates_path is stated explicitly rather than inherited from gp_sphinx's identical default, so a later change to that default cannot silently stop the search override from loading.

docs/_static/libtmux-org.css: @imports the shared tokens.css and translates --lt-* onto Furo's --color-* for light, data-theme="dark", and prefers-color-scheme: dark. Its header carries the full rationale.

docs/_templates/search.html: replaces the theme's search page outright — no {% extends %}. The wording matches the rediraffe stubs already in this tree so the site audit's redirect check skips it instead of flagging an empty page.

pyproject.toml: declares sphinx-gp-llms directly in dev and docs so [tool.uv.sources] can redirect it — a source override does not reach a dependency that arrives transitively.

Design decisions

Import the tokens, never copy them. The values stay live at the CDN, so a chrome-color fix reaches an already-published build without a rebuild here. Only the mapping — which --lt-* name feeds which --color-* name — is fixed at build time.

Every var(--lt-*) carries Furo's own stock value as its fallback. This is what makes the file repetitive, and it is load-bearing: a var() naming an undefined custom property with no fallback resolves to the guaranteed-invalid value, so --color-background-primary would compute to unset rather than white. Any page whose cross-origin tokens.css fetch 404s would then render worse than the glitch this file closes. The fallbacks are Furo's stock colors rather than libtmux's, so a failed fetch degrades to plain Furo instead of vendoring the palette.

Override sphinx-gp-llms alone, not the gp-sphinx meta-package. Pinning the meta-package makes uv resolve its siblings from the fork too, and they are published at a pinned version the fork does not carry.

Test plan

Not run before opening; uv sync needs a key for the fork above.

• uv sync --all-extras --dev resolves
• cd docs && just html builds clean
• Built pages list libtmux-org.css after css/custom.css, and load /_shell/shell.js
• The built search.html redirects to /search/
• With tokens.css unreachable, pages render as stock Furo in light and dark