fix(runner): name the file the bind proof creates instead of walking a tree

`Io.Dir` has no `deleteTreeAbsolute` — the teardown called one that does not
exist, and every Zig lane that compiles the runner INTEGRATION binary failed
on it: lint-zig, test-unit-agentsfleetd, test-coverage-zig,
test-integration-kernel, and the `test` aggregator behind them.

It compiled locally because `zig build test` builds the runner UNIT binary and
this file rides the integration one, so nothing local ever saw it. Verified
this time against the target that failed — `test-integration-bin` and
`check-test-reachability`, both exit 0.

The teardown now names the single file the child creates rather than sweeping
recursively from an interpolated path — the test wrote that directory's whole
contents, so it can say what to remove.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>