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/wisp/io.go

@@ -30,12 +30,12 @@ func writeJSON(path string, v interface{}) error {
 
 	// Write to temp file then rename for atomicity
 	tmp := path + ".tmp"
-	if err := os.WriteFile(tmp, data, 0644); err != nil {
+	if err := os.WriteFile(tmp, data, 0644); err != nil { //nolint:gosec // G306: wisp messages are non-sensitive operational data
 		return fmt.Errorf("write temp: %w", err)
 	}
 
 	if err := os.Rename(tmp, path); err != nil {
-		os.Remove(tmp) // cleanup on failure
+		_ = os.Remove(tmp) // cleanup on failure
 		return fmt.Errorf("rename: %w", err)
 	}