Phone runtime: agents can't reliably use /sdcard

Agents on the phone's in-app runtime (#11) can't reliably use phone storage at /sdcard.

What we know

Setup asks for All files access (Settings > Environments > this phone > phone setup > Phone storage). The runtime binds /storage into Ubuntu and makes /sdcard a symlink to /storage/emulated/0, so that a grant made after the server starts takes effect without a restart.
On a Galaxy Z Fold5, after the grant, an agent's write test in /sdcard succeeded, but [ -w /sdcard ] reported false. Agents that test before writing conclude storage isn't writable.
Android's shared storage keeps no permissions, ownership, symlinks or hard links. So git, npm, tar -p and cp -a fail there even when plain writes work.
The emulator test can't check storage: run-as processes can't read shared storage even with the grant. Only the app's own processes can, so this needs a check on a real phone.

The exact failure on the phone: the command, its error, and whether the Phone storage row in setup showed access as allowed.
Whether proot's view of /storage is the cause (for example the bind, the symlink or FUSE permission checks), or the expected limits of shared storage.
What the agent guide (apps/mobile/modules/t3-device-runtime/android/src/main/assets/phone-agent-guide.md) should tell agents.