Improve settings UX with enriched schema descriptions and walkthrough

[dollfins]

Description

The extension currently relies entirely on settings.json for configuration. While this is standard for VS Code extensions, providing a dedicated settings webview or using VS Code's native settings UI more effectively would improve discoverability and ease of use.

Current State

• sorobanGuard.path — string input (must know the path)
• sorobanGuard.severity — enum dropdown
• sorobanGuard.runOnSave — boolean toggle
• sorobanGuard.exclude — string (comma-separated globs)

Users must know these exist or discover them through the README.

Expected Behavior

1. Improve existing settings schema in package.json:

   • Add markdownDescription with examples for each setting
   • Add markdownEnumDescriptions for severity levels explaining what each shows/hides
   • Add editPresentation for the exclude pattern (multi-line string input)

2. Add a settings walkthrough using VS Code's walkthroughs contribution point:

   • Step 1: Check CLI binary is installed
   • Step 2: Run first scan
   • Step 3: Configure severity levels
   • Step 4: View report panel

3. Add a welcome view in the activity bar when no Rust file is open:

   • Quick actions: Scan Workspace, Open Settings, View Docs

Implementation Notes

• Settings schema improvements are purely package.json changes
• Walkthroughs use the walkthroughs contribution point in package.json
• Welcome view uses a TreeDataProvider if needed

Acceptance Criteria

• Each setting has a markdownDescription with practical examples
• Severity enum has markdownEnumDescriptions explaining filtering behavior
• Walkthrough is accessible from VS Code's "Get Started" page
• Walkthrough steps work correctly end-to-end

Complexity

Medium — requires understanding of VS Code contribution points and walkthrough API.

Points

150