fix(create): use agent-aware prefix extraction for agent beads

The generic ValidateIDFormat() used isLikelyHash() which treated
3-character suffixes like "nux" as valid hashes, causing agent IDs
like "nx-nexus-polecat-nux" to extract prefix as "nx-nexus-polecat"
instead of the correct "nx".

Fix: For --type=agent, validate agent ID format first and use
ExtractAgentPrefix() which correctly extracts prefix from the
first hyphen for agent IDs.

Fixes #591

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

internal/validation/bead_test.go

@@ -404,3 +404,43 @@ func TestValidateAgentID(t *testing.T) {
 		})
 	}
 }
+
+func TestExtractAgentPrefix(t *testing.T) {
+	tests := []struct {
+		name       string
+		id         string
+		wantPrefix string
+	}{
+		// Town-level agents
+		{"mayor", "gt-mayor", "gt"},
+		{"deacon", "gt-deacon", "gt"},
+		{"bd mayor", "bd-mayor", "bd"},
+
+		// Per-rig agents
+		{"witness", "gt-gastown-witness", "gt"},
+		{"refinery", "bd-beads-refinery", "bd"},
+
+		// Named agents - the bug case
+		{"polecat 3-char name", "nx-nexus-polecat-nux", "nx"},
+		{"polecat regular", "gt-gastown-polecat-phoenix", "gt"},
+		{"crew", "gt-beads-crew-dave", "gt"},
+
+		// Hyphenated rig names
+		{"hyphenated rig", "gt-my-project-witness", "gt"},
+		{"multi-hyphen rig polecat", "bd-my-cool-app-polecat-bob", "bd"},
+
+		// Edge cases
+		{"no hyphen", "nohyphen", ""},
+		{"empty", "", ""},
+		{"just prefix", "gt-", "gt"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ExtractAgentPrefix(tt.id)
+			if got != tt.wantPrefix {
+				t.Errorf("ExtractAgentPrefix(%q) = %q, want %q", tt.id, got, tt.wantPrefix)
+			}
+		})
+	}
+}