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/tui/feed/mq_source.go

@@ -35,7 +35,7 @@ func NewMQEventSource(beadsDir string) (*MQEventSource, error) {
 		if err != nil {
 			return nil, err
 		}
-		f.Close()
+		_ = f.Close() //nolint:gosec // G104: best-effort close on file creation
 	}
 
 	file, err := os.Open(logPath)
@@ -71,7 +71,7 @@ func (s *MQEventSource) tail(ctx context.Context) {
 	defer close(s.events)
 
 	// Seek to end for live tailing
-	s.file.Seek(0, 2)
+	_, _ = s.file.Seek(0, 2)
 
 	scanner := bufio.NewScanner(s.file)
 	ticker := time.NewTicker(100 * time.Millisecond)