#15953: fix: Android/Termux node fails to discover commands

## Summary This PR fixes the issue where Android/Termux nodes connect successfully but fail to execute any commands due to an empty commands array. The root cause was that the Android platform default allowlist in `node-command-policy.ts` did not include `SYSTEM_COMMANDS` (containing `system.run`, `system.which`, etc.). When a node connects, the gateway filters the node's declared commands against the platform allowlist, and since these commands were missing from Android's defaults, the filtered list became empty. ## Changes - Added `...SYSTEM_COMMANDS` to the Android platform defaults in `PLATFORM_DEFAULTS` in `src/gateway/node-command-policy.ts` - This aligns Android with other command-capable platforms (macos, linux, windows) that already include `SYSTEM_COMMANDS` ## Testing - Ran existing unit tests: `npx vitest run src/gateway/node-command-policy.test.ts` - 2 tests pass - Ran gateway test suite: `npx vitest run --config vitest.gateway.config.ts` - 315 tests pass - Linting passes with no warnings Fixes openclaw/openclaw#15944