Pkg-cards: Hugo partial + shortcode + color-coded categories (garden#5)

Ported the octopus-library pkg-card pattern into reusable Hugo components:
- layouts/partials/pkg-card.html — renders data-card="pkg" with category
- layouts/shortcodes/pkg-card.html — markdown-usable shortcode
- garden.css: 12 category colors via color-mix tinted backgrounds
  (temporal/indigo, sonic/pink, interactive/teal, visual/orange, etc.)

Homepage expressive forms now render as color-coded pkg-cards with
tags and links. Each category gets a left border accent + tinted pill.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

layouts/shortcodes/pkg-card.html

@@ -0,0 +1,14 @@
+{{ $tags := slice }}
+{{ with .Get "tags" }}
+  {{ $tags = split . "," }}
+{{ end }}
+
+{{ partial "pkg-card.html" (dict
+  "name"        (.Get "name")
+  "category"    (.Get "category")
+  "description" (.Get "description")
+  "tags"        $tags
+  "link"        (.Get "link")
+  "reaches"     (.Get "reaches")
+  "version"     (.Get "version")
+) }}