fix: correct manifest.json to pass MCPB validation

- Add required 'version' field (1.0.0)
- Change 'author' from string to object with name/url
- Move 'mcp_config' inside 'server' object
- Fix 'user_config' to use proper field-level structure
- Remove invalid 'compatibility' and 'capabilities' top-level fields
- Create bundle icon (512x512 PNG) to satisfy icon requirement
- Remove icon placeholder file

Validation: ✅ mcpb validate passes with warnings only
Tasks: T149 (icon), T158 (validate), T159 (build) marked complete

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

assets/icon.png.placeholder

@@ -1,12 +0,0 @@
-# Icon Placeholder
-# 
-# This is a placeholder for the bundle icon.
-# To complete this task:
-# 1. Create a 512x512 PNG icon representing the Obsidian MCP Bundle
-# 2. Place it at assets/icon.png
-# 3. Remove this placeholder file
-#
-# Suggested icon concepts:
-# - Obsidian logo combined with AI/connection symbols
-# - Brain/network icon in Obsidian's purple color scheme
-# - Document/note icon with chat/conversation elements