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/dog/manager.go

@@ -509,7 +509,7 @@ func (m *Manager) saveState(name string, state *DogState) error {
 		return err
 	}
 
-	return os.WriteFile(m.stateFilePath(name), data, 0644)
+	return os.WriteFile(m.stateFilePath(name), data, 0644) //nolint:gosec // G306: dog state is non-sensitive operational data
 }
 
 // GetIdleDog returns an idle dog suitable for work assignment.