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/connection/local.go

@@ -33,7 +33,7 @@ func (c *LocalConnection) IsLocal() bool {
 
 // ReadFile reads the named file.
 func (c *LocalConnection) ReadFile(path string) ([]byte, error) {
-	data, err := os.ReadFile(path)
+	data, err := os.ReadFile(path) //nolint:gosec // G304: path is from Connection interface, validated by caller
 	if err != nil {
 		if os.IsNotExist(err) {
 			return nil, &NotFoundError{Path: path}