docs(sidebar[captions]): show package tier captions in global sidebar

why: The three package tier captions (Shared Infrastructure, Domain
Packages, Theme/Fonts/CLI) were defined in docs/packages/index.md but
did not render in the Furo sidebar — Sphinx's toctree function only
emits <p class="caption"> separators for top-level captioned toctrees
defined in the root document.
what:
- Move the three captioned :hidden: toctrees from docs/packages/index.md
  to docs/index.md so they become top-level navigation groups
- Remove the now-duplicate captioned toctrees from packages/index.md
  (the page keeps its narrative tier description and workspace grid)
- Sidebar now shows three labelled groups instead of a flat list of 12

Author: tony

docs/index.md

@@ -97,3 +97,33 @@ api
 project/index
 history
 ```
+
+```{toctree}
+:caption: Shared Infrastructure
+:hidden:
+
+packages/sphinx-ux-badges
+packages/sphinx-ux-autodoc-layout
+packages/sphinx-autodoc-typehints-gp
+```
+
+```{toctree}
+:caption: Domain Packages
+:hidden:
+
+packages/sphinx-autodoc-api-style
+packages/sphinx-autodoc-docutils
+packages/sphinx-autodoc-fastmcp
+packages/sphinx-autodoc-pytest-fixtures
+packages/sphinx-autodoc-sphinx
+```
+
+```{toctree}
+:caption: Theme, Fonts & CLI
+:hidden:
+
+packages/gp-sphinx
+packages/sphinx-gp-theme
+packages/sphinx-fonts
+packages/sphinx-autodoc-argparse
+```

docs/packages/index.md

@@ -28,33 +28,3 @@ and FastMCP tools all look like they belong together.
 
 ```{workspace-package-grid}
 ```
-
-```{toctree}
-:caption: Shared Infrastructure
-:hidden:
-
-sphinx-ux-badges
-sphinx-ux-autodoc-layout
-sphinx-autodoc-typehints-gp
-```
-
-```{toctree}
-:caption: Domain Packages
-:hidden:
-
-sphinx-autodoc-api-style
-sphinx-autodoc-docutils
-sphinx-autodoc-fastmcp
-sphinx-autodoc-pytest-fixtures
-sphinx-autodoc-sphinx
-```
-
-```{toctree}
-:caption: Theme, Fonts & CLI
-:hidden:
-
-gp-sphinx
-sphinx-gp-theme
-sphinx-fonts
-sphinx-autodoc-argparse
-```