fix: improve rig name validation error message

- Use underscores instead of stripping invalid characters
- Convert suggestion to lowercase for consistency
- Explicitly state that underscores are allowed

Before: "MyProject.jl" → "MyProjectjl"
After:  "MyProject.jl" → "myproject_jl"

Closes #97

Co-Authored-By: Olivier Debeuf De Rijcker <markov-kernel@users.noreply.github.com>

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

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

internal/rig/manager.go

@@ -212,8 +212,9 @@ func (m *Manager) AddRig(opts AddRigOptions) (*Rig, error) {
 	// Validate rig name: reject characters that break agent ID parsing
 	// Agent IDs use format <prefix>-<rig>-<role>[-<name>] with hyphens as delimiters
 	if strings.ContainsAny(opts.Name, "-. ") {
-		sanitized := strings.NewReplacer("-", "", ".", "", " ", "").Replace(opts.Name)
-		return nil, fmt.Errorf("rig name %q contains invalid characters (hyphens, dots, or spaces break agent ID parsing); use %q instead", opts.Name, sanitized)
+		sanitized := strings.NewReplacer("-", "_", ".", "_", " ", "_").Replace(opts.Name)
+		sanitized = strings.ToLower(sanitized)
+		return nil, fmt.Errorf("rig name %q contains invalid characters; hyphens, dots, and spaces are reserved for agent ID parsing. Try %q instead (underscores are allowed)", opts.Name, sanitized)
 	}
 
 	rigPath := filepath.Join(m.townRoot, opts.Name)