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   - checkAndSuggestBDGuideUpdate() detects outdated BD_GUIDE.md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_test.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"bytes"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -68,3 +70,120 @@ func TestOnboardCommand(t *testing.T) {
 		}
 	})
 }
+
+func TestGenerateBDGuide(t *testing.T) {
+	t.Run("generates BD_GUIDE.md with version stamp", func(t *testing.T) {
+		// Create temp directory
+		tmpDir := t.TempDir()
+		outputPath := filepath.Join(tmpDir, "BD_GUIDE.md")
+
+		// Generate BD_GUIDE.md
+		if err := generateBDGuide(outputPath); err != nil {
+			t.Fatalf("generateBDGuide() error = %v", err)
+		}
+
+		// Read generated file
+		content, err := os.ReadFile(outputPath)
+		if err != nil {
+			t.Fatalf("Failed to read generated file: %v", err)
+		}
+
+		output := string(content)
+
+		// Verify version stamp in header
+		if !strings.Contains(output, "Auto-generated by bd v"+Version) {
+			t.Error("Generated file should contain version stamp in header")
+		}
+
+		if !strings.Contains(output, "DO NOT EDIT MANUALLY") {
+			t.Error("Generated file should contain DO NOT EDIT warning")
+		}
+
+		// Verify regeneration instructions
+		if !strings.Contains(output, "bd onboard --output") {
+			t.Error("Generated file should contain regeneration instructions")
+		}
+	})
+
+	t.Run("includes agents content", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		outputPath := filepath.Join(tmpDir, "BD_GUIDE.md")
+
+		if err := generateBDGuide(outputPath); err != nil {
+			t.Fatalf("generateBDGuide() error = %v", err)
+		}
+
+		content, err := os.ReadFile(outputPath)
+		if err != nil {
+			t.Fatalf("Failed to read generated file: %v", err)
+		}
+
+		output := string(content)
+
+		// Verify key sections from agentsContent are present
+		expectedSections := []string{
+			"Issue Tracking with bd (beads)",
+			"bd ready",
+			"bd create",
+			"MCP Server",
+		}
+
+		for _, section := range expectedSections {
+			if !strings.Contains(output, section) {
+				t.Errorf("Generated file should contain '%s'", section)
+			}
+		}
+	})
+
+	t.Run("includes copilot instructions content", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		outputPath := filepath.Join(tmpDir, "BD_GUIDE.md")
+
+		if err := generateBDGuide(outputPath); err != nil {
+			t.Fatalf("generateBDGuide() error = %v", err)
+		}
+
+		content, err := os.ReadFile(outputPath)
+		if err != nil {
+			t.Fatalf("Failed to read generated file: %v", err)
+		}
+
+		output := string(content)
+
+		// Verify key sections from copilotInstructionsContent are present
+		expectedSections := []string{
+			"GitHub Copilot Instructions",
+			"Project Structure",
+			"Tech Stack",
+			"Coding Guidelines",
+		}
+
+		for _, section := range expectedSections {
+			if !strings.Contains(output, section) {
+				t.Errorf("Generated file should contain '%s'", section)
+			}
+		}
+	})
+
+	t.Run("has proper structure with separators", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		outputPath := filepath.Join(tmpDir, "BD_GUIDE.md")
+
+		if err := generateBDGuide(outputPath); err != nil {
+			t.Fatalf("generateBDGuide() error = %v", err)
+		}
+
+		content, err := os.ReadFile(outputPath)
+		if err != nil {
+			t.Fatalf("Failed to read generated file: %v", err)
+		}
+
+		output := string(content)
+
+		// Count separators (should have at least 3: after header, between sections, before footer)
+		separatorCount := strings.Count(output, "---")
+		if separatorCount < 3 {
+			t.Errorf("Expected at least 3 separators (---), got %d", separatorCount)
+		}
+	})
+}