docs: add per-tool metadata feature to Tools Screen

Address cliffhall's feedback about tool-specific metadata:
- Add [Edit Metadata] button for per-tool _meta fields
- Stored per-server, per-tool (not in global localStorage)
- Merged with server-level metadata on each call

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

specification/v2_ux.md

@@ -559,6 +559,10 @@ Each feature screen uses a **resizable panel layout** for flexibility.
   - Dropdown suggestions as user types
   - Supports enum and dynamic completion
 - Form generated from tool input schema
+- **Per-Tool Metadata** - [Edit Metadata] button to set tool-specific `_meta` fields:
+  - Stored per-server, per-tool (not in global localStorage)
+  - Merged with server-level metadata on each call
+  - Use case: Different progressToken behavior, tool-specific tracking IDs
 - **Progress Indicator** from `notifications/progress`:
   - Progress bar with percentage
   - Step description if provided