refactor: use _hash instead of -hash in GCS path

[ValentaTomas]

Aligns the kernel GCS path with fc-versions' `_<short_hash>` convention (e.g. `v1.14.1_458ca91`):

```
gs:///vmlinux-_<short_hash>//vmlinux.bin
```

3 spots touched: script header comment + the `dst=` line in `scripts/upload-release-to-gcs.sh` + the README example.

Verified with `--dry-run`:

```
WOULD vmlinux-6.1.102-amd64.bin -> gs://e2b-staging-fc-kernels/vmlinux-6.1.102_c1a568c/amd64/vmlinux.bin
```

Note: anything already uploaded with the old dash path is not auto-renamed; future uploads use the underscore.

[cursor]

PR Summary

Low Risk
Low risk because this is a small, mechanical path change, but it can break consumers or manual lookups that still expect the old -<short_hash> layout and it does not migrate existing objects.

Overview
Changes the GCS destination path used by scripts/upload-release-to-gcs.sh (and the README example) from vmlinux-<version>-<short_hash> to vmlinux-<version>_<short_hash>.

Anything already uploaded under the old dash-separated prefix will not be found/used by the updated path unless downstream tooling is updated to match.

^{Reviewed by Cursor Bugbot for commit fdb81fb. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ValentaTomas]

Folding into #22 — the underscore change rides along with the bash 3.2 fix there.