Remote stack imports that use the go-getter `//subdir` notation produce a malformed `git clone` URL

### Describe the Bug

## Description

Remote stack imports that use the go-getter `//subdir` notation produce a malformed `git clone` URL in v1.220.0-rc.3 (the first release containing #2037). The subdir portion is dropped, and the repository name is also stripped, so `git` is ultimately invoked against `https://github.com//` — the repo name and subdir path missing.

Both `git::https://...` and `github.com/...` shorthand forms fail the same way. Public and private source repos both reproduce.

## Expected Behavior

Per the [docs](https://atmos.tools/stacks/imports#remote-imports) and the [`examples/remote-stack-imports`](https://github.com/cloudposse/atmos/tree/main/examples/remote-stack-imports) example, the URL below should clone `cloudposse/atmos@main` and merge `examples/quick-start-simple/stacks/orgs/acme/plat/dev.yaml`:

```yaml
import:
- git::https://github.com/cloudposse/atmos.git//examples/quick-start-simple/stacks/orgs/acme/plat/dev?ref=main
```

## Steps to Reproduce (public-repo, self-contained)

`atmos.yaml`:
```yaml
version:
use: "1.220.0-rc.3"
base_path: "."
components:
terraform:
base_path: components/terraform
stacks:
base_path: stacks
included_paths: ["**/*"]
name_pattern: "{environment}-{stage}"
```

`stacks/dev.yaml`:
```yaml
import:
- git::https://github.com/cloudposse/atmos.git//examples/quick-start-simple/stacks/orgs/acme/plat/dev?ref=main

vars:
namespace: acme
tenant: plat
environment: ue1
stage: services-dev-01
```

Run: `atmos describe stacks -s ue1-services-dev-01`

## Actual Behavior

```
Error: failed to download remote import
'git::https://github.com/cloudposse/atmos.git//examples/quick-start-simple/stacks/orgs/acme/plat/dev?ref=main'
in file 'dev.yaml': failed to download remote import: failed to fetch content
for cache: failed to download file: /usr/bin/git exited with 128: Cloning into
'/.../getter.../temp'...
remote: Not Found
fatal: repository 'https://github.com/cloudposse/' not found
```

Note the URL git received: `https://github.com/cloudposse/` — both the repo name (`atmos.git`) and the `//examples/...` subdir are missing.

Debug-log excerpt from a token-injected (private-repo) variant — note the subdir is already absent from the very first detector.

The subdir portion (`//stacks/mixins/region/us-east-1`) present at `Download` time is gone by the time CustomGitDetector logs its input — suggesting the source URL is correctly separated from the subdir, but the subdir is then dropped, never re-appended, **and** the final clone command is further mangled to omit the repo name itself.

## Environment

- atmos: v1.220.0-rc.3 (asdf-managed install on darwin/arm64; also reproduces in Linux GHA runner)
- macOS 25.5.0
- Reproduces with both public (`cloudposse/atmos`) and private source repos
- Reproduces with `git::https://` explicit form **and** `github.com/...` shorthand
- HTTPS file URL form (`https://raw.githubusercontent.com/owner/repo/branch/file.yaml`) works for public files (no auth path for private repos)

## Additional Context

- First release containing #2037 / closing #2036; new `pkg/stack/imports/` package and `pkg/downloader/custom_git_detector.go` are in this code path.
- Happy to test a candidate patch against the same scenario.

### Expected Behavior

_No response_

### Steps to Reproduce

_No response_

### Screenshots

_No response_

### Environment

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### Additional Context

_No response_