feat: auto-protect forks from committing upstream issue database

When bd detects it's running in a fork (origin != steveyegge/beads),
automatically add .beads/issues.jsonl to .git/info/exclude.

This prevents contributors from accidentally including issue database
changes in their PRs. The exclusion is:
- Per-clone (doesn't modify tracked files
- One-time setup (checks if already excluded)
- Silent (only logs in debug mode)

Maintainers (origin = steveyegge/beads) are not affected.

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

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

cmd/bd/fork_protection_test.go

@@ -0,0 +1,113 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestIsUpstreamRepo(t *testing.T) {
+	tests := []struct {
+		name     string
+		remote   string
+		expected bool
+	}{
+		{"ssh upstream", "git@github.com:steveyegge/beads.git", true},
+		{"https upstream", "https://github.com/steveyegge/beads.git", true},
+		{"https upstream no .git", "https://github.com/steveyegge/beads", true},
+		{"fork ssh", "git@github.com:contributor/beads.git", false},
+		{"fork https", "https://github.com/contributor/beads.git", false},
+		{"different repo", "git@github.com:someone/other-project.git", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Verify the pattern matching logic matches what isUpstreamRepo uses
+			upstreamPatterns := []string{
+				"steveyegge/beads",
+				"git@github.com:steveyegge/beads",
+				"https://github.com/steveyegge/beads",
+			}
+
+			matches := false
+			for _, pattern := range upstreamPatterns {
+				if strings.Contains(tt.remote, pattern) {
+					matches = true
+					break
+				}
+			}
+
+			if matches != tt.expected {
+				t.Errorf("remote %q: expected upstream=%v, got %v", tt.remote, tt.expected, matches)
+			}
+		})
+	}
+}
+
+func TestIsAlreadyExcluded(t *testing.T) {
+	// Create temp file with exclusion
+	tmpDir := t.TempDir()
+	excludePath := filepath.Join(tmpDir, "exclude")
+
+	// Test non-existent file
+	if isAlreadyExcluded(excludePath) {
+		t.Error("expected non-existent file to return false")
+	}
+
+	// Test file without exclusion
+	if err := os.WriteFile(excludePath, []byte("*.log\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if isAlreadyExcluded(excludePath) {
+		t.Error("expected file without exclusion to return false")
+	}
+
+	// Test file with exclusion
+	if err := os.WriteFile(excludePath, []byte("*.log\n.beads/issues.jsonl\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if !isAlreadyExcluded(excludePath) {
+		t.Error("expected file with exclusion to return true")
+	}
+}
+
+func TestAddToExclude(t *testing.T) {
+	tmpDir := t.TempDir()
+	infoDir := filepath.Join(tmpDir, ".git", "info")
+	excludePath := filepath.Join(infoDir, "exclude")
+
+	// Test creating new file
+	if err := addToExclude(excludePath); err != nil {
+		t.Fatalf("addToExclude failed: %v", err)
+	}
+
+	content, err := os.ReadFile(excludePath)
+	if err != nil {
+		t.Fatalf("failed to read exclude file: %v", err)
+	}
+
+	if !strings.Contains(string(content), ".beads/issues.jsonl") {
+		t.Errorf("exclude file missing .beads/issues.jsonl: %s", content)
+	}
+
+	// Test appending to existing file
+	if err := os.WriteFile(excludePath, []byte("*.log\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := addToExclude(excludePath); err != nil {
+		t.Fatalf("addToExclude append failed: %v", err)
+	}
+
+	content, err = os.ReadFile(excludePath)
+	if err != nil {
+		t.Fatalf("failed to read exclude file: %v", err)
+	}
+
+	if !strings.Contains(string(content), "*.log") {
+		t.Errorf("exclude file missing original content: %s", content)
+	}
+	if !strings.Contains(string(content), ".beads/issues.jsonl") {
+		t.Errorf("exclude file missing .beads/issues.jsonl: %s", content)
+	}
+}