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/formula/parser.go

@@ -9,7 +9,7 @@ import (
 
 // ParseFile reads and parses a formula.toml file.
 func ParseFile(path string) (*Formula, error) {
-	data, err := os.ReadFile(path)
+	data, err := os.ReadFile(path) //nolint:gosec // G304: path is from trusted formula directory
 	if err != nil {
 		return nil, fmt.Errorf("reading formula file: %w", err)
 	}