feat(init): Auto-detect fork repos and offer to configure .git/info/exclude (GH#742)

When bd init runs in a forked repo (detected via upstream remote), prompt
the user to configure .git/info/exclude to keep beads files local.

- Add --setup-exclude flag for manual trigger
- Add fork detection to init flow (reuses detectForkSetup)
- Add setupForkExclude() to configure .git/info/exclude with:
  .beads/, **/RECOVERY*.md, **/SESSION*.md
- Add promptForkExclude() with Y/n prompt (default yes)
- Add containsExactPattern() helper for precise pattern matching

🤖 Generated with Claude Code

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

cmd/bd/init.go

@@ -426,6 +426,24 @@ With --stealth: configures per-repository git settings for invisible beads usage
 			fmt.Fprintf(os.Stderr, "Warning: failed to close database: %v\n", err)
 		}
 
+		// Fork detection: offer to configure .git/info/exclude (GH#742)
+		setupExclude, _ := cmd.Flags().GetBool("setup-exclude")
+		if setupExclude {
+			// Manual flag - always configure
+			if err := setupForkExclude(!quiet); err != nil {
+				fmt.Fprintf(os.Stderr, "Warning: failed to configure git exclude: %v\n", err)
+			}
+		} else if !stealth && isGitRepo() {
+			// Auto-detect fork and prompt (skip if stealth - it handles exclude already)
+			if isFork, upstreamURL := detectForkSetup(); isFork {
+				if promptForkExclude(upstreamURL, quiet) {
+					if err := setupForkExclude(!quiet); err != nil {
+						fmt.Fprintf(os.Stderr, "Warning: failed to configure git exclude: %v\n", err)
+					}
+				}
+			}
+		}
+
 		// Check if we're in a git repo and hooks aren't installed
 		// Install by default unless --skip-hooks is passed
 		if !skipHooks && isGitRepo() && !hooksInstalled() {
@@ -491,6 +509,7 @@ func init() {
 	initCmd.Flags().Bool("contributor", false, "Run OSS contributor setup wizard")
 	initCmd.Flags().Bool("team", false, "Run team workflow setup wizard")
 	initCmd.Flags().Bool("stealth", false, "Enable stealth mode: global gitattributes and gitignore, no local repo tracking")
+	initCmd.Flags().Bool("setup-exclude", false, "Configure .git/info/exclude to keep beads files local (for forks)")
 	initCmd.Flags().Bool("skip-hooks", false, "Skip git hooks installation")
 	initCmd.Flags().Bool("skip-merge-driver", false, "Skip git merge driver setup")
 	initCmd.Flags().Bool("force", false, "Force re-initialization even if JSONL already has issues (may cause data loss)")
@@ -1463,6 +1482,103 @@ func setupGitExclude(verbose bool) error {
 	return nil
 }
 
+// setupForkExclude configures .git/info/exclude for fork workflows (GH#742)
+// Adds beads files and Claude artifacts to keep PRs to upstream clean.
+// This is separate from stealth mode - fork protection is specifically about
+// preventing beads/Claude files from appearing in upstream PRs.
+func setupForkExclude(verbose bool) error {
+	gitDir, err := exec.Command("git", "rev-parse", "--git-dir").Output()
+	if err != nil {
+		return fmt.Errorf("not a git repository")
+	}
+	gitDirPath := strings.TrimSpace(string(gitDir))
+	excludePath := filepath.Join(gitDirPath, "info", "exclude")
+
+	// Ensure info directory exists
+	if err := os.MkdirAll(filepath.Join(gitDirPath, "info"), 0755); err != nil {
+		return fmt.Errorf("failed to create git info directory: %w", err)
+	}
+
+	// Read existing content
+	var existingContent string
+	// #nosec G304 - git config path
+	if content, err := os.ReadFile(excludePath); err == nil {
+		existingContent = string(content)
+	}
+
+	// Patterns to add for fork protection
+	patterns := []string{".beads/", "**/RECOVERY*.md", "**/SESSION*.md"}
+	var toAdd []string
+	for _, p := range patterns {
+		// Check for exact line match (pattern alone on a line)
+		// This avoids false positives like ".beads/issues.jsonl" matching ".beads/"
+		if !containsExactPattern(existingContent, p) {
+			toAdd = append(toAdd, p)
+		}
+	}
+
+	if len(toAdd) == 0 {
+		if verbose {
+			fmt.Printf("%s Git exclude already configured\n", ui.RenderPass("✓"))
+		}
+		return nil
+	}
+
+	// Append patterns
+	newContent := existingContent
+	if !strings.HasSuffix(newContent, "\n") && len(newContent) > 0 {
+		newContent += "\n"
+	}
+	newContent += "\n# Beads fork protection (bd init)\n"
+	for _, p := range toAdd {
+		newContent += p + "\n"
+	}
+
+	// #nosec G306 - config file needs 0644
+	if err := os.WriteFile(excludePath, []byte(newContent), 0644); err != nil {
+		return fmt.Errorf("failed to write git exclude: %w", err)
+	}
+
+	if verbose {
+		fmt.Printf("\n%s Added to .git/info/exclude:\n", ui.RenderPass("✓"))
+		for _, p := range toAdd {
+			fmt.Printf("  %s\n", p)
+		}
+		fmt.Println("\nNote: .git/info/exclude is local-only and won't affect upstream.")
+	}
+	return nil
+}
+
+// containsExactPattern checks if content contains the pattern as an exact line
+// This avoids false positives like ".beads/issues.jsonl" matching ".beads/"
+func containsExactPattern(content, pattern string) bool {
+	for _, line := range strings.Split(content, "\n") {
+		if strings.TrimSpace(line) == pattern {
+			return true
+		}
+	}
+	return false
+}
+
+// promptForkExclude asks if user wants to configure .git/info/exclude for fork workflow (GH#742)
+func promptForkExclude(upstreamURL string, quiet bool) bool {
+	if quiet {
+		return false // Don't prompt in quiet mode
+	}
+
+	fmt.Printf("\n%s Detected fork (upstream: %s)\n\n", ui.RenderAccent("▶"), upstreamURL)
+	fmt.Println("Would you like to configure .git/info/exclude to keep beads files local?")
+	fmt.Println("This prevents beads from appearing in PRs to upstream.")
+	fmt.Print("\n[Y/n]: ")
+
+	reader := bufio.NewReader(os.Stdin)
+	response, _ := reader.ReadString('\n')
+	response = strings.TrimSpace(strings.ToLower(response))
+
+	// Default to yes (empty or "y" or "yes")
+	return response == "" || response == "y" || response == "yes"
+}
+
 // setupGlobalGitIgnore configures global gitignore to ignore beads and claude files for a specific project
 // DEPRECATED: This function uses absolute paths which don't work in gitignore (GitHub #704).
 // Use setupGitExclude instead for new code.