test(tui): delivery_tests sandbox case fails when the workspace lives under /tmp (bwrap --tmpfs /tmp shadows it)

## Observed (deterministic, isolation)

On the CI lane (x86_64 Linux), the sandbox delivery test fails whenever the test workspace is a `tempfile` under `/tmp`:

```
cargo test -p codewhale-tui --lib --locked \
tools::subagent::delivery_tests::enforced_readonly_python_queries_sqlite_under_a_live_peer_write_claim -- --exact

panicked: called `Result::unwrap()` on an `Err` value:
Failed to execute tool: bwrap: Can't chdir to /tmp/.tmpN8FjYQ: No such file or directory
```

## Root cause

`crates/tui/src/sandbox/bwrap.rs` mounts `--tmpfs /tmp` and then binds the workspace roots. A workspace that itself lives under `/tmp` is shadowed by the tmpfs, so the subsequent `--chdir ` targets a path that no longer exists inside the sandbox.

## Pre-existing?

Yes — it fails identically at the pre-change control commit for the current fleet work, and the code path is untouched by it. It is a test-environment interaction, not a product regression.

## Suggested fix

Test-side: create the sandbox fixture workspace outside `/tmp` on Linux (the crate already knows the sandbox layout). Do not weaken the `--tmpfs /tmp` isolation, which is deliberate.

Seen during the 2026-09-17 fleet-rework lane runs; reported by the Codewhale night program.

Run the exact cargo test command for tools::subagent::delivery_tests::enforced_readonly_python_queries_sqlite_under_a_live_peer_write_claim, then inspect its sandbox fixture and crates/tui/src/sandbox/bwrap.rs. On Linux, place the test workspace outside /tmp while preserving the deliberate --tmpfs /tmp isolation. Done means the exact test passes in the affected environment.

Written by the indexing model from the issue text.