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.go

@@ -0,0 +1,99 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"github.com/steveyegge/beads/internal/debug"
+)
+
+// ensureForkProtection prevents contributors from accidentally committing
+// the upstream issue database when working in a fork.
+//
+// When we detect this is a fork (origin != steveyegge/beads), we add
+// .beads/issues.jsonl to .git/info/exclude so it won't be staged.
+// This is a per-clone setting that doesn't modify tracked files.
+func ensureForkProtection() {
+	// Find git root (reuses existing findGitRoot from autoimport.go)
+	gitRoot := findGitRoot()
+	if gitRoot == "" {
+		return // Not in a git repo
+	}
+
+	// Check if this is the upstream repo (maintainers)
+	if isUpstreamRepo(gitRoot) {
+		return // Maintainers can commit issues.jsonl
+	}
+
+	// Check if already excluded
+	excludePath := filepath.Join(gitRoot, ".git", "info", "exclude")
+	if isAlreadyExcluded(excludePath) {
+		return
+	}
+
+	// Add to .git/info/exclude
+	if err := addToExclude(excludePath); err != nil {
+		debug.Printf("fork protection: failed to update exclude: %v", err)
+		return
+	}
+
+	debug.Printf("Fork detected: .beads/issues.jsonl excluded from git staging")
+}
+
+// isUpstreamRepo checks if origin remote points to the upstream beads repo
+func isUpstreamRepo(gitRoot string) bool {
+	cmd := exec.Command("git", "-C", gitRoot, "remote", "get-url", "origin")
+	out, err := cmd.Output()
+	if err != nil {
+		return false // Can't determine, assume fork for safety
+	}
+
+	remote := strings.TrimSpace(string(out))
+
+	// Check for upstream repo patterns
+	upstreamPatterns := []string{
+		"steveyegge/beads",
+		"git@github.com:steveyegge/beads",
+		"https://github.com/steveyegge/beads",
+	}
+
+	for _, pattern := range upstreamPatterns {
+		if strings.Contains(remote, pattern) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// isAlreadyExcluded checks if issues.jsonl is already in the exclude file
+func isAlreadyExcluded(excludePath string) bool {
+	content, err := os.ReadFile(excludePath) //nolint:gosec // G304: path is constructed from git root, not user input
+	if err != nil {
+		return false // File doesn't exist or can't read, not excluded
+	}
+
+	return strings.Contains(string(content), ".beads/issues.jsonl")
+}
+
+// addToExclude adds the issues.jsonl pattern to .git/info/exclude
+func addToExclude(excludePath string) error {
+	// Ensure the directory exists
+	dir := filepath.Dir(excludePath)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return err
+	}
+
+	// Open for append (create if doesn't exist)
+	f, err := os.OpenFile(excludePath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644) //nolint:gosec // G302: .git/info/exclude should be world-readable
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	// Add our exclusion with a comment
+	_, err = f.WriteString("\n# Beads: prevent fork from committing upstream issue database\n.beads/issues.jsonl\n")
+	return err
+}