## Reproduction
An installed adapter has `[update] avenue = "gh_release"`, repo `BigscreenVR/omp-spt`, asset `adapter.spt`. The latest stable GitHub release tag is `omp-spt-v0.2.0`.

```console
spt adapter update omp-spt
```

spt 0.31.0 logs discovery of the correct release:

```text
ADAPTER_UPDATE:omp-spt: 0.1.3 -> omp-spt-v0.2.0
```

A PATH shim around public `gh` captures the actual child argv:

```text
release download vomp-spt-v0.2.0 --repo BigscreenVR/omp-spt --pattern adapter.spt --dir ... --clobber
```

The extra leading `v` makes the fetch fail. Direct `gh release download omp-spt-v0.2.0 ...` succeeds.

## Expected
Use the exact tag returned by GitHub release discovery. Do not synthesize `v{tag}` after a complete tag has already been discovered.

## Impact
Adapters whose canonical release tags include a project prefix can be installed with an explicit `adapter add --release ... --tag ...`, but cannot self-update through the documented `gh_release` avenue.