fix: Address golangci-lint errors (errcheck, gosec) (#76)

Apply PR #76 from dannomayernotabot:

- Add golangci exclusions for internal package false positives
- Tighten file permissions (0644 -> 0600) for sensitive files
- Add ReadHeaderTimeout to HTTP server (slowloris prevention)
- Explicit error ignoring with _ = for intentional cases
- Add //nolint comments with justifications
- Spelling: cancelled -> canceled (US locale)

Co-Authored-By: dannomayernotabot <noreply@github.com>

🤖 Generated with Claude Code

internal/cmd/molecule_step.go

@@ -250,7 +250,7 @@ func findNextReadyStep(b *beads.Beads, moleculeID string) (*beads.Issue, bool, e
 }
 
 // handleStepContinue handles continuing to the next step.
-func handleStepContinue(cwd, townRoot, workDir string, nextStep *beads.Issue, dryRun bool) error {
+func handleStepContinue(cwd, townRoot, _ string, nextStep *beads.Issue, dryRun bool) error { // workDir unused but kept for signature consistency
 	fmt.Printf("\n%s Next step: %s\n", style.Bold.Render("→"), nextStep.ID)
 	fmt.Printf("  %s\n", nextStep.Title)