Sandbox: report the extra temp-storage read-write rule in sandbox.decision, and allow opting out #5365

When a host passes a sandbox config with exactly one read-write path, the resolved policy reports two read-write rules (sandbox.decision / policy_resolved shows readwrite: 2). The second one is the temporary-storage grant, which follows $TMPDIR. It is not named in the event, and the schema's readwritePaths does not mention it either; the schema only says temporary storage "follows its own rules and stays granted".

Repro (Linux, bubblewrap, Copilot CLI 1.0.105; also seen with the SDK 1.0.24 runtime):

1. Start a session with a sandbox config whose only read-write path is the project folder, addCurrentWorkingDirectory: false, allowDevToolAccess: false.
2. Approve touch "$TMPDIR/x": it succeeds. A sibling folder outside the project is refused, as expected.
3. The sandbox.decision event lists readwrite: 2 without saying what the second rule is.

Requests:

1. Include the resolved rule paths (or at least the temp-storage path) in sandbox.decision, so a host can show the user exactly what is writable.
2. Offer an opt-out for the temp-storage grant (for example allowTempStorage: false), for hosts that want a strict policy and can give the session its own scratch folder.