Switch from hex to Base36 encoding for issue IDs (GH #213)

This change improves information density by using Base36 (0-9, a-z) instead
of hex (0-9, a-f) for hash-based issue IDs. Key benefits:

- Shorter IDs: Can now use 3-char IDs (was 4-char minimum)
- Better scaling: 3 chars good for ~160 issues, 4 chars for ~980 issues
- Case-insensitive: Maintains excellent CLI usability
- Backward compatible: Old hex IDs continue to work

Changes:
- Implemented Base36 encoding with proper truncation (keep LSB)
- Updated adaptive length thresholds (3-8 chars instead of 4-8)
- Fixed collision probability math to match encoding (was calculating
  for base36 but encoding in hex - now both use base36)
- Fixed ID parser bug (use prefixWithHyphen for substring matching)
- Updated all tests and test data patterns

Fixes #213

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

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

internal/utils/id_parser.go

@@ -78,11 +78,11 @@ func ResolvePartialID(ctx context.Context, store storage.Storage, input string)
 	}
 	
 	// Extract the hash part for substring matching
-	hashPart := strings.TrimPrefix(normalizedID, prefix)
-	
+	hashPart := strings.TrimPrefix(normalizedID, prefixWithHyphen)
+
 	var matches []string
 	for _, issue := range issues {
-		issueHash := strings.TrimPrefix(issue.ID, prefix)
+		issueHash := strings.TrimPrefix(issue.ID, prefixWithHyphen)
 		// Check if the issue hash contains the input hash as substring
 		if strings.Contains(issueHash, hashPart) {
 			matches = append(matches, issue.ID)