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/issue.go

@@ -122,10 +122,7 @@ func detectCurrentSession() string {
 
 	// Check if we're mayor
 	if os.Getenv("GT_ROLE") == "mayor" {
-		mayorSession, err := getMayorSessionName()
-		if err == nil {
-			return mayorSession
-		}
+		return getMayorSessionName()
 	}
 
 	return ""