fix: correct plugin install command syntax in docs (v1.4.1)

The Claude Code plugin installation is a two-step process per the official
docs at code.claude.com/docs/en/plugin-marketplaces. There is no
'claude plugin install github:owner/repo' form. Users must first add the
marketplace catalog, then install the plugin from it by name.

Fixes:

* README.md: Option 2 install section now shows the two-step flow:
    claude plugin marketplace add AgriciDaniel/claude-obsidian
    claude plugin install claude-obsidian@claude-obsidian-marketplace

* docs/install-guide.md: Same correction, same syntax.

* plugin.json: 1.4.0 to 1.4.1
* marketplace.json: 1.4.0 to 1.4.1 (both metadata.version and
  plugins[0].version)

This fixes the reported "Plugin not found in any configured marketplace"
error users see when they try the old single-command form.

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

README.md

@@ -52,12 +52,23 @@ Open Claude Code in the same folder. Type `/wiki`.
 
 ### Option 2: Install as Claude Code plugin
 
+Plugin installation is a two-step process in Claude Code. First add the marketplace catalog, then install the plugin from it.
+
 ```bash
-claude plugin install github:AgriciDaniel/claude-obsidian
+# Step 1: add the marketplace
+claude plugin marketplace add AgriciDaniel/claude-obsidian
+
+# Step 2: install the plugin
+claude plugin install claude-obsidian@claude-obsidian-marketplace
 ```
 
 In any Claude Code session: `/wiki`. Claude walks you through vault setup.
 
+To check it worked:
+```bash
+claude plugin list
+```
+
 ---
 
 ### Option 3: Add to an existing vault