feat(prime): add --state, --dry-run, --explain flags with mutual exclusivity validation

Add three new flags to gt prime command:
- --state: Output role state as JSON and exit early (for scripting)
- --dry-run: Skip side effects (persistence, locks, events)
- --explain: Show verbose role detection reasoning

The --state flag is mutually exclusive with all other flags and errors
if combined. The other flags (--dry-run, --explain, --hook) can be
combined freely.

Also fixes missing filepath import in beads.go.

Closes: bd-t8ven

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

internal/cmd/prime_test.go

@@ -2,7 +2,9 @@ package cmd
 
 import (
 	"os"
+	"os/exec"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/steveyegge/gastown/internal/beads"
@@ -95,3 +97,73 @@ func TestGetAgentBeadID_UsesRigPrefix(t *testing.T) {
 		})
 	}
 }
+
+func TestPrimeFlagCombinations(t *testing.T) {
+	// Find the gt binary - we need to test CLI flag validation
+	gtBin, err := exec.LookPath("gt")
+	if err != nil {
+		t.Skip("gt binary not found in PATH")
+	}
+
+	cases := []struct {
+		name      string
+		args      []string
+		wantError bool
+		errorMsg  string
+	}{
+		{
+			name:      "state_alone_is_valid",
+			args:      []string{"prime", "--state"},
+			wantError: false, // May fail for other reasons (not in workspace), but not flag validation
+		},
+		{
+			name:      "state_with_hook_errors",
+			args:      []string{"prime", "--state", "--hook"},
+			wantError: true,
+			errorMsg:  "--state cannot be combined with other flags",
+		},
+		{
+			name:      "state_with_dry_run_errors",
+			args:      []string{"prime", "--state", "--dry-run"},
+			wantError: true,
+			errorMsg:  "--state cannot be combined with other flags",
+		},
+		{
+			name:      "state_with_explain_errors",
+			args:      []string{"prime", "--state", "--explain"},
+			wantError: true,
+			errorMsg:  "--state cannot be combined with other flags",
+		},
+		{
+			name:      "dry_run_and_explain_valid",
+			args:      []string{"prime", "--dry-run", "--explain"},
+			wantError: false, // May fail for other reasons, but not flag validation
+		},
+		{
+			name:      "hook_and_dry_run_valid",
+			args:      []string{"prime", "--hook", "--dry-run"},
+			wantError: false, // May fail for other reasons, but not flag validation
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(gtBin, tc.args...)
+			output, err := cmd.CombinedOutput()
+
+			if tc.wantError {
+				if err == nil {
+					t.Fatalf("expected error, got success with output: %s", output)
+				}
+				if tc.errorMsg != "" && !strings.Contains(string(output), tc.errorMsg) {
+					t.Fatalf("expected error containing %q, got: %s", tc.errorMsg, output)
+				}
+			}
+			// For non-error cases, we don't fail on other errors (like "not in workspace")
+			// because we're only testing flag validation
+			if !tc.wantError && tc.errorMsg != "" && strings.Contains(string(output), tc.errorMsg) {
+				t.Fatalf("unexpected error message %q in output: %s", tc.errorMsg, output)
+			}
+		})
+	}
+}