chore(config): add Obsidian MCP server and bin plugin

## Why
The Obsidian MCP server (mcpvault) wasn't wired up in this repo's local
config, making obsidian tools unavailable. The bin@holocron plugin was
added to settings but not yet listed in the allowed plugins.

## Impact
- Obsidian MCP tools are now available when working in this repo
- bin@holocron plugin is enabled in .claude/settings.json

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.mcp.json

@@ -0,0 +1,12 @@
+{
+  "mcpServers": {
+    "obsidian": {
+      "type": "stdio",
+      "command": "npx",
+      "args": [
+        "@bitbonsai/mcpvault@latest",
+        "docs/"
+      ]
+    }
+  }
+}