Add vulnerable example and enhance action features

This commit adds a comprehensive example demonstrating the govulncheck action's capabilities, along with several enhancements to improve the user experience.

## Changes

### Example Setup
- Added example/main.go that uses golang.org/x/net v0.0.0-20220906165146-f3363e06e74c (vulnerable version)
- Example intentionally calls html.Parse to trigger vulnerability detection
- Demonstrates how the action creates annotations on vulnerable code

### Enhanced Annotations
- Fixed file path handling when running in subdirectories (e.g., './example')
- Added rich context to annotations including:
  - Vulnerability summaries and CVE numbers
  - Direct links to Go vulnerability database (pkg.go.dev/vuln)
  - Fixed version information
  - Clear indication of which function is vulnerable (e.g., "html.Parse" not "main")
- Sorted vulnerabilities by OSV ID for consistent display

### Suggested Fixes
- When vulnerable code is actually called, creates notice annotations on go.mod
- Shows current vs suggested dependency versions
- Lists which specific vulnerabilities have active call sites
- Provides actionable upgrade recommendations

### Workflow Summary
- Added comprehensive workflow summary using GitHub Actions summary API
- Displays vulnerabilities in formatted tables by module
- Shows vulnerable code locations organized by file
- Includes links to Go vulnerability database entries
- Provides clear recommendations for fixing vulnerabilities

### Bug Fixes
- Fixed JSON parsing to handle both JSON lines and multi-line JSON formats
- Fixed annotation file paths to be relative to repository root
- Improved vulnerability function name extraction from trace data
- Enhanced OSV detail parsing and storage

## Testing
The example workflow demonstrates all features by intentionally using a vulnerable dependency. When run, it will:
1. Detect vulnerabilities in golang.org/x/net
2. Create warning annotations on go.mod and main.go
3. Suggest specific version updates
4. Generate a detailed workflow summary

This provides a complete demonstration of the action's vulnerability detection and reporting capabilities.

.github/workflows/example.yml

@@ -25,23 +25,6 @@ jobs:
         with:
           working-directory: './example'
       
-      - name: Report vulnerability scan results
-        if: always()
-        run: |
-          echo "## Vulnerability Scan Results"
-          echo "- Vulnerabilities found: ${{ steps.govulncheck.outputs.vulnerabilities-found }}"
-          echo "- Vulnerability count: ${{ steps.govulncheck.outputs.vulnerability-count }}"
-          
-          if [ "${{ steps.govulncheck.outputs.vulnerabilities-found }}" == "true" ]; then
-            echo ""
-            echo "⚠️ Security vulnerabilities were detected in the example code."
-            echo "This is expected! The example intentionally uses a vulnerable version of golang.org/x/net"
-            echo "to demonstrate how the action creates annotations."
-          else
-            echo ""
-            echo "✅ No vulnerabilities found (this would be unexpected for the example)."
-          fi
-      
       # Note: We don't fail the workflow here since the example is intentionally vulnerable
       - name: Demonstrate conditional failure
         run: |