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/claude/settings.go

@@ -67,7 +67,7 @@ func EnsureSettings(workDir string, roleType RoleType) error {
 	}
 
 	// Write settings file
-	if err := os.WriteFile(settingsPath, content, 0644); err != nil {
+	if err := os.WriteFile(settingsPath, content, 0600); err != nil {
 		return fmt.Errorf("writing settings: %w", err)
 	}