Correct the source type for the marketplace itself

The previous commit changed the settings snippet to "url" on the grounds
that no "git" type exists. That reasoning conflated two things the
documentation deliberately separates: where the catalogue comes from and
where an individual plugin comes from. The list of github/url/git-subdir
applies to the second. Adding the marketplace for real settled it —
Claude Code wrote "git" into known_marketplaces.json itself.

So the snippet says "git" again, now with the HTTPS URL that works, and
spells out the distinction rather than leaving the next person to trip
over it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -57,15 +57,22 @@ project's `.claude/settings.json`:
 {
   "extraKnownMarketplaces": {
     "pdat": {
-      "source": { "source": "url", "url": "https://git.pda.cz/PDAT/claude-plugins.git" }
+      "source": { "source": "git", "url": "https://git.pda.cz/PDAT/claude-plugins.git" }
     }
   },
   "enabledPlugins": { "pdat-swift-practice@pdat": true }
 }
 ```
 
-The git-based source types are `github`, `url` and `git-subdir`; there is
-no `git` type. For a self-hosted host, `url` is the one.
+Two different things are called a source, and they take different types:
+
+- **Marketplace source** — where the catalogue itself comes from. That is
+  the block above, and for a git repository the type is `git`. It is what
+  `/plugin marketplace add` writes into `known_marketplaces.json`.
+- **Plugin source** — where an individual plugin listed in
+  `marketplace.json` comes from: `github`, `url`, `git-subdir`, `npm`,
+  `archive` or `command`. Ours is a relative path into this repository, so
+  it needs none of them.
 
 Anyone who clones that project gets the skill without installing anything.