Add bd preflight command with static checklist (Phase 1)

Scaffolds bd preflight for PR readiness checks. Currently prints a
static checklist of common pre-PR validation items:
- Tests pass
- Lint passes
- No beads pollution
- Nix hash current
- Version sync

--check and --fix flags are documented but not yet implemented
(see bd-lfak.2 through bd-lfak.5 for roadmap).

(bd-lfak.1)

Cherry-picked from polecat/amber-1767146829144

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

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

cmd/bd/preflight.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+var preflightCmd = &cobra.Command{
+	Use:     "preflight",
+	GroupID: "maint",
+	Short:   "Show PR readiness checklist",
+	Long: `Display a checklist of common pre-PR checks for contributors.
+
+This command helps catch common issues before pushing to CI:
+- Tests not run locally
+- Lint errors
+- Stale nix vendorHash
+- Version mismatches
+
+Phase 1 shows a static checklist. Future phases will add:
+- --check: Run checks automatically
+- --fix: Auto-fix where possible
+
+Examples:
+  bd preflight              # Show checklist
+  bd preflight --check      # (future) Run checks automatically
+  bd preflight --fix        # (future) Auto-fix where possible
+`,
+	Run: runPreflight,
+}
+
+func init() {
+	// Future flags (documented but not yet implemented)
+	preflightCmd.Flags().Bool("check", false, "Run checks automatically (not yet implemented)")
+	preflightCmd.Flags().Bool("fix", false, "Auto-fix issues where possible (not yet implemented)")
+
+	rootCmd.AddCommand(preflightCmd)
+}
+
+func runPreflight(cmd *cobra.Command, args []string) {
+	// Check for future flags
+	check, _ := cmd.Flags().GetBool("check")
+	fix, _ := cmd.Flags().GetBool("fix")
+
+	if check || fix {
+		fmt.Println("Note: --check and --fix are not yet implemented.")
+		fmt.Println("See bd-lfak.2 through bd-lfak.5 for implementation roadmap.")
+		fmt.Println()
+	}
+
+	fmt.Println("PR Readiness Checklist:")
+	fmt.Println()
+	fmt.Println("[ ] Tests pass: go test -short ./...")
+	fmt.Println("[ ] Lint passes: golangci-lint run ./...")
+	fmt.Println("[ ] No beads pollution: check .beads/issues.jsonl diff")
+	fmt.Println("[ ] Nix hash current: go.sum unchanged or vendorHash updated")
+	fmt.Println("[ ] Version sync: version.go matches default.nix")
+	fmt.Println()
+	fmt.Println("Run 'bd preflight --check' to validate automatically (coming soon).")
+}