docs(api) Visual improvements via gp-sphinx (#10)

Adopt the gp-sphinx Sphinx package stack for libtmux-mcp's API
documentation. The net visual effect is card-style autodoc
signatures with safety and scope badges, MyST cross-reference
roles for Python objects, scoped section names in multi-page CLI
docs, and polished IBM Plex typography — all landing together from
the gp-sphinx 0.0.1a7 pre-release.

- **API-style autodoc layout** via sphinx-autodoc-api-style —
  card layouts, badges, MyST roles for Python objects
- **Shared badge layer** via sphinx-autodoc-badges — consistent
  XS/SM/LG/XL size variants with WCAG-AA contrast
- **Multi-page argparse scoping** via sphinx-argparse-neo — CLI
  doc builds no longer emit duplicate-label warnings
  (git-pull/gp-sphinx#16)
- **FastMCP autodoc** via sphinx-autodoc-fastmcp — card-style
  desc layouts for FastMCP tools, safety badges, MyST directives
- **Font polish** via sphinx-fonts — IBM Plex Sans and Mono at
  full weight range, zero-CLS loading

See gp-sphinx 0.0.1a7 release notes for the full list of changes
covering 0.0.1a1..0.0.1a7:
https://github.com/git-pull/gp-sphinx/blob/v0.0.1a7/CHANGES

Author: tony

CHANGES

@@ -1,5 +1,15 @@
 # Changelog
 
+## libtmux-mcp 0.1.x (unreleased)
+
+<!-- KEEP THIS PLACEHOLDER - DO NOT REMOVE OR MODIFY THIS LINE -->
+_Notes on upcoming releases will be added here_
+<!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
+
+### Documentation
+
+- Visual improvements to API docs from [gp-sphinx](https://gp-sphinx.git-pull.com)-based Sphinx packages (#10)
+
 ## libtmux-mcp 0.1.0a0 (2026-03-22)
 
 ### New features