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/events.go

@@ -255,7 +255,7 @@ func (s *GtEventsSource) 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)