fix(test): anchor all test sandboxes on reclaimable pid-free names

Every hand-rolled test-sandbox helper keyed its directory on
std::process::id() under env::temp_dir(), so the pre-create wipe never
matched a prior run's path: one directory stranded per helper site per
test-binary invocation, >50k dirs / 15.5 GiB on the 16 GiB tmpfs by
2026-07-13.

The remedy extends commit 84e1075's knob-lab pattern to every site:
fixed, tag-keyed, PID-FREE names under the build-tree tmp anchor, each
site keeping its pre-create remove_dir_all — which now genuinely
reclaims the previous run. Integration/bench targets use
env!("CARGO_TARGET_TMPDIR"); src-internal unit-test helpers (cargo sets
that var only for integration/bench targets) derive the same anchor
from env!("CARGO_MANIFEST_DIR") + ../../target/tmp. Residue is bounded
to one directory per site, on disk instead of the tmpfs, per checkout.

One deliberate exception: project_new.rs's tmp() stays under
env::temp_dir() because two of its tests (new_outside_a_work_tree_*)
need a base genuinely detached from any git work tree, and target/
lives inside the checkout's work tree. Its name carries a per-checkout
hash discriminator instead, so concurrently running checkouts (which
share /tmp) cannot race on the fixed name while each run still
reclaims its predecessor.

tempfile-RAII was considered and rejected (issue #258 decision log):
it cannot cover the process-lifetime OnceLock scaffolds (statics never
drop), adds a dependency, and still strands kill-leftovers on the
tmpfs.

Verified: full workspace suite green (incl. the new RED regression
test temp_cwd_sandbox_does_not_leak_under_env_temp_dir), clippy clean,
sandboxes observed under target/tmp with stable names across runs.
Stale pre-fix PID-keyed dirs in /tmp are not swept by this change and
age out only by manual cleanup.

closes #258

crates/aura-cli/tests/cli_run.rs

@@ -15,7 +15,7 @@ const BIN: &str = env!("CARGO_BIN_EXE_aura");
 /// records (so `aura sweep`'s `./runs/runs.jsonl` never dirties the repo).
 /// Unique per test + per process; no external tempfile dependency.
 fn temp_cwd(name: &str) -> std::path::PathBuf {
-    let dir = std::env::temp_dir().join(format!("aura-cli-{}-{}", std::process::id(), name));
+    let dir = Path::new(env!("CARGO_TARGET_TMPDIR")).join(format!("aura-cli-{name}"));
     let _ = std::fs::remove_dir_all(&dir);
     std::fs::create_dir_all(&dir).expect("create temp cwd");
     dir