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>

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-obsidian",
-  "version": "1.4.0",
+  "version": "1.4.1",
   "description": "Claude + Obsidian knowledge companion. Sets up a persistent, compounding wiki vault. Covers memory management, session notetaking, knowledge organization, and agent context across projects. Based on Andrej Karpathy's LLM Wiki pattern.",
   "author": {
     "name": "AgriciDaniel",