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/tmux/theme.go

@@ -63,7 +63,7 @@ func AssignThemeFromPalette(rigName string, palette []Theme) Theme {
 		return DefaultPalette[0]
 	}
 	h := fnv.New32a()
-	h.Write([]byte(rigName))
+	_, _ = h.Write([]byte(rigName))
 	idx := int(h.Sum32()) % len(palette)
 	return palette[idx]
 }