feat: add agent ID pattern validation for bd create --type=agent (gt-hlaaf)

- Update ParseIssueType to use canonical types.IsValid() (includes agent, role, gate, message)
- Add ValidateAgentID function to validate gt-<role>[-<rig>[-<name>]] pattern
- Wire up validation in create.go when --type=agent with explicit ID
- Update --type flag help text to include agent and role types
- Add comprehensive test coverage for agent ID validation

internal/validation/bead.go

@@ -30,18 +30,8 @@ func ParsePriority(content string) int {
 func ParseIssueType(content string) (types.IssueType, error) {
 	issueType := types.IssueType(strings.TrimSpace(content))
 
-	// Validate issue type
-	validTypes := map[types.IssueType]bool{
-		types.TypeBug:          true,
-		types.TypeFeature:      true,
-		types.TypeTask:         true,
-		types.TypeEpic:         true,
-		types.TypeChore:        true,
-		types.TypeMergeRequest: true,
-		types.TypeMolecule:     true,
-	}
-
-	if !validTypes[issueType] {
+	// Use the canonical IsValid() from types package
+	if !issueType.IsValid() {
 		return types.TypeTask, fmt.Errorf("invalid issue type: %s", content)
 	}
 
@@ -88,3 +78,100 @@ func ValidatePrefix(requestedPrefix, dbPrefix string, force bool) error {
 
 	return fmt.Errorf("prefix mismatch: database uses '%s' but you specified '%s' (use --force to override)", dbPrefix, requestedPrefix)
 }
+
+// ValidAgentRoles are the known agent role types for ID pattern validation
+var ValidAgentRoles = []string{
+	"mayor",    // Town-level: gt-mayor
+	"deacon",   // Town-level: gt-deacon
+	"witness",  // Per-rig: gt-witness-<rig>
+	"refinery", // Per-rig: gt-refinery-<rig>
+	"crew",     // Per-rig with name: gt-crew-<rig>-<name>
+	"polecat",  // Per-rig with name: gt-polecat-<rig>-<name>
+}
+
+// TownLevelRoles are agent roles that don't have a rig
+var TownLevelRoles = []string{"mayor", "deacon"}
+
+// NamedRoles are agent roles that include a worker name
+var NamedRoles = []string{"crew", "polecat"}
+
+// ValidateAgentID validates that an agent ID follows the expected pattern.
+// Patterns:
+//   - Town-level: gt-<role> (e.g., gt-mayor, gt-deacon)
+//   - Per-rig: gt-<role>-<rig> (e.g., gt-witness-gastown)
+//   - Named: gt-<role>-<rig>-<name> (e.g., gt-polecat-gastown-nux)
+//
+// Returns nil if the ID is valid, or an error describing the issue.
+func ValidateAgentID(id string) error {
+	if id == "" {
+		return fmt.Errorf("agent ID is required")
+	}
+
+	// Must start with gt-
+	if !strings.HasPrefix(id, "gt-") {
+		return fmt.Errorf("agent ID must start with 'gt-' (got %q)", id)
+	}
+
+	// Split into parts after the prefix
+	rest := id[3:] // Skip "gt-"
+	parts := strings.Split(rest, "-")
+	if len(parts) < 1 || parts[0] == "" {
+		return fmt.Errorf("agent ID must include role type: gt-<role>[-<rig>[-<name>]] (got %q)", id)
+	}
+
+	role := parts[0]
+
+	// Check if role is valid
+	validRole := false
+	for _, r := range ValidAgentRoles {
+		if role == r {
+			validRole = true
+			break
+		}
+	}
+	if !validRole {
+		return fmt.Errorf("invalid agent role %q (valid: %s)", role, strings.Join(ValidAgentRoles, ", "))
+	}
+
+	// Check town-level roles (no rig allowed)
+	for _, r := range TownLevelRoles {
+		if role == r {
+			if len(parts) > 1 {
+				return fmt.Errorf("town-level agent %q cannot have rig suffix (expected gt-%s, got %q)", role, role, id)
+			}
+			return nil // Valid town-level agent
+		}
+	}
+
+	// Per-rig agents require at least a rig
+	if len(parts) < 2 {
+		return fmt.Errorf("per-rig agent %q requires rig: gt-%s-<rig> (got %q)", role, role, id)
+	}
+
+	rig := parts[1]
+	if rig == "" {
+		return fmt.Errorf("rig name cannot be empty in %q", id)
+	}
+
+	// Check named roles (require name)
+	for _, r := range NamedRoles {
+		if role == r {
+			if len(parts) < 3 {
+				return fmt.Errorf("agent %q requires name: gt-%s-<rig>-<name> (got %q)", role, role, id)
+			}
+			name := parts[2]
+			if name == "" {
+				return fmt.Errorf("agent name cannot be empty in %q", id)
+			}
+			// Extra parts after name are allowed (e.g., for complex identifiers)
+			return nil // Valid named agent
+		}
+	}
+
+	// Regular per-rig agents (witness, refinery) - should have exactly 2 parts
+	if len(parts) > 2 {
+		return fmt.Errorf("agent %q takes only rig: gt-%s-<rig> (got %q)", role, role, id)
+	}
+
+	return nil // Valid per-rig agent
+}