feat: add version sync check to bd preflight --check (bd-lfak.5)

- Implement runVersionSyncCheck() to compare version.go and default.nix
- Reports version mismatch as hard failure
- Gracefully skips if default.nix is missing
- Fix version mismatch: update default.nix from 0.37.0 to 0.42.0

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/preflight.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"regexp"
 	"strings"
 
 	"github.com/spf13/cobra"
@@ -99,6 +100,10 @@ func runChecks(jsonOutput bool) {
 	nixResult := runNixHashCheck()
 	results = append(results, nixResult)
 
+	// Run version sync check
+	versionResult := runVersionSyncCheck()
+	results = append(results, versionResult)
+
 	// Calculate overall result
 	allPassed := true
 	passCount := 0
@@ -244,6 +249,82 @@ func runNixHashCheck() CheckResult {
 	}
 }
 
+// runVersionSyncCheck checks that version.go matches default.nix.
+func runVersionSyncCheck() CheckResult {
+	command := "Compare cmd/bd/version.go and default.nix"
+
+	// Read version.go
+	versionGoContent, err := os.ReadFile("cmd/bd/version.go")
+	if err != nil {
+		return CheckResult{
+			Name:    "Version sync",
+			Passed:  false,
+			Skipped: true,
+			Output:  fmt.Sprintf("Cannot read cmd/bd/version.go: %v", err),
+			Command: command,
+		}
+	}
+
+	// Extract version from version.go
+	// Pattern: Version = "X.Y.Z"
+	versionGoRe := regexp.MustCompile(`Version\s*=\s*"([^"]+)"`)
+	versionGoMatch := versionGoRe.FindSubmatch(versionGoContent)
+	if versionGoMatch == nil {
+		return CheckResult{
+			Name:    "Version sync",
+			Passed:  false,
+			Skipped: true,
+			Output:  "Cannot parse version from version.go",
+			Command: command,
+		}
+	}
+	goVersion := string(versionGoMatch[1])
+
+	// Read default.nix
+	nixContent, err := os.ReadFile("default.nix")
+	if err != nil {
+		// No nix file = skip version check (not an error)
+		return CheckResult{
+			Name:    "Version sync",
+			Passed:  true,
+			Skipped: true,
+			Output:  "default.nix not found (skipping nix version check)",
+			Command: command,
+		}
+	}
+
+	// Extract version from default.nix
+	// Pattern: version = "X.Y.Z";
+	nixRe := regexp.MustCompile(`version\s*=\s*"([^"]+)"`)
+	nixMatch := nixRe.FindSubmatch(nixContent)
+	if nixMatch == nil {
+		return CheckResult{
+			Name:    "Version sync",
+			Passed:  false,
+			Skipped: true,
+			Output:  "Cannot parse version from default.nix",
+			Command: command,
+		}
+	}
+	nixVersion := string(nixMatch[1])
+
+	if goVersion != nixVersion {
+		return CheckResult{
+			Name:    "Version sync",
+			Passed:  false,
+			Output:  fmt.Sprintf("Version mismatch: version.go=%s, default.nix=%s", goVersion, nixVersion),
+			Command: command,
+		}
+	}
+
+	return CheckResult{
+		Name:    "Version sync",
+		Passed:  true,
+		Output:  fmt.Sprintf("Versions match: %s", goVersion),
+		Command: command,
+	}
+}
+
 // truncateOutput truncates output to maxLen characters, adding ellipsis if truncated.
 func truncateOutput(s string, maxLen int) string {
 	if len(s) <= maxLen {