Implement BD_GUIDE.md generation for version-stamped documentation (bd-woro)

This implements the ability to separate bd-specific instructions from
project-specific instructions by generating a canonical BD_GUIDE.md file.

## Changes

1. Added `--output` flag to `bd onboard` command
   - Generates version-stamped BD_GUIDE.md at specified path
   - Includes both agentsContent and copilotInstructionsContent
   - Auto-generated header warns against manual editing

2. Version tracking integration
   - checkAndSuggestBDGuideUpdate() detects outdated BD_GUIDE.md
   - Suggests regeneration when bd version changes
   - Integrated with maybeShowUpgradeNotification()

3. Comprehensive test coverage
   - Tests for BD_GUIDE.md generation
   - Tests for version stamp validation
   - Tests for content inclusion

4. Documentation updates
   - Updated AGENTS.md with BD_GUIDE.md workflow
   - Added regeneration instructions to upgrade workflow

## Benefits

- Clear separation of concerns (bd vs project instructions)
- Deterministic updates (no LLM involved)
- Git-trackable diffs show exactly what changed
- Progressive disclosure (agents read when needed)

## Usage

\`\`\`bash
# Generate BD_GUIDE.md
bd onboard --output .beads/BD_GUIDE.md

# After upgrading bd
bd onboard --output .beads/BD_GUIDE.md  # Regenerate
\`\`\`

Closes bd-woro

cmd/bd/onboard.go

@@ -415,6 +415,66 @@ func renderOnboardInstructions(w io.Writer) error {
 	return nil
 }
 
+// bdGuideContent generates the canonical BD_GUIDE.md content
+const bdGuideHeader = `<!-- Auto-generated by bd v%s - DO NOT EDIT MANUALLY -->
+<!-- Run 'bd onboard --output .beads/BD_GUIDE.md' to regenerate -->
+
+# BD (Beads) Guide for AI Agents
+
+This file contains canonical bd (beads) workflow instructions for AI agents.
+It is auto-generated and version-stamped to track bd upgrades.
+
+> **For project-specific instructions**, see AGENTS.md in the repository root.
+> This file only covers bd tool usage, not project-specific workflows.
+
+---
+
+`
+
+// generateBDGuide creates a version-stamped BD_GUIDE.md file
+func generateBDGuide(outputPath string) error {
+	// Create output file
+	f, err := os.Create(outputPath)
+	if err != nil {
+		return fmt.Errorf("failed to create output file: %w", err)
+	}
+	defer f.Close()
+
+	// Write header with version stamp
+	if _, err := fmt.Fprintf(f, bdGuideHeader, Version); err != nil {
+		return fmt.Errorf("failed to write header: %w", err)
+	}
+
+	// Write AGENTS.md content (bd-specific instructions)
+	if _, err := f.WriteString(agentsContent); err != nil {
+		return fmt.Errorf("failed to write agents content: %w", err)
+	}
+
+	// Write separator
+	if _, err := f.WriteString("\n\n---\n\n"); err != nil {
+		return fmt.Errorf("failed to write separator: %w", err)
+	}
+
+	// Write Copilot instructions content (comprehensive technical guide)
+	if _, err := f.WriteString(copilotInstructionsContent); err != nil {
+		return fmt.Errorf("failed to write copilot content: %w", err)
+	}
+
+	// Write footer with regeneration instructions
+	footer := fmt.Sprintf("\n\n---\n\n"+
+		"**Generated by bd v%s**\n\n"+
+		"To regenerate this file after upgrading bd:\n"+
+		"```bash\n"+
+		"bd onboard --output .beads/BD_GUIDE.md\n"+
+		"```\n", Version)
+
+	if _, err := f.WriteString(footer); err != nil {
+		return fmt.Errorf("failed to write footer: %w", err)
+	}
+
+	return nil
+}
+
 var onboardCmd = &cobra.Command{
 	Use:   "onboard",
 	Short: "Display instructions for configuring AGENTS.md",
@@ -422,8 +482,29 @@ var onboardCmd = &cobra.Command{
 
 This command outputs instructions that AI agents should follow to integrate bd
 into the project's agent documentation. The agent will intelligently merge the
-content into AGENTS.md and update CLAUDE.md if present.`,
+content into AGENTS.md and update CLAUDE.md if present.
+
+Use --output to generate a canonical BD_GUIDE.md file instead:
+  bd onboard --output .beads/BD_GUIDE.md
+
+The generated BD_GUIDE.md is version-stamped and auto-generated - it should
+never be manually edited. This separates bd-specific instructions (which change
+with bd upgrades) from project-specific instructions in AGENTS.md.`,
 	Run: func(cmd *cobra.Command, args []string) {
+		outputPath, _ := cmd.Flags().GetString("output")
+
+		if outputPath != "" {
+			// Generate BD_GUIDE.md instead of onboarding instructions
+			if err := generateBDGuide(outputPath); err != nil {
+				fmt.Fprintf(cmd.ErrOrStderr(), "Error generating BD_GUIDE.md: %v\n", err)
+				os.Exit(1)
+			}
+			fmt.Printf("✓ Generated %s (bd v%s)\n", outputPath, Version)
+			fmt.Println("  This file is auto-generated - do not edit manually")
+			fmt.Println("  Update your AGENTS.md to reference this file instead of duplicating bd instructions")
+			return
+		}
+
 		if err := renderOnboardInstructions(cmd.OutOrStdout()); err != nil {
 			if _, writeErr := fmt.Fprintf(cmd.ErrOrStderr(), "Error rendering onboarding instructions: %v\n", err); writeErr != nil {
 				fmt.Fprintf(os.Stderr, "Error rendering onboarding instructions: %v (stderr write failed: %v)\n", err, writeErr)
@@ -434,5 +515,6 @@ content into AGENTS.md and update CLAUDE.md if present.`,
 }
 
 func init() {
+	onboardCmd.Flags().String("output", "", "Generate BD_GUIDE.md at the specified path (e.g., .beads/BD_GUIDE.md)")
 	rootCmd.AddCommand(onboardCmd)
 }