[BUG] SSH-registered github.com marketplace: in-repo plugin pre-flight goes over HTTPS only, so a private marketplace cannot install without a token #3164

Describe the bug

For a marketplace on github.com registered over SSH (ssh://git@github.com/<org>/<repo>.git or
git@github.com:<org>/<repo>.git), apm install <plugin>@<marketplace> for an in-marketplace subdirectory plugin
validates (and tries to fetch) the plugin over HTTPS — raw.githubusercontent.com, the Contents API on
api.github.com, then an anonymous git ls-remote https://github.com/<org>/<repo>.git — and never over SSH. With no
GitHub token available (SSH-key-only environment: no GITHUB_TOKEN/GH_TOKEN, no gh on PATH, no credential helper),
a private marketplace therefore cannot install any of its in-repo plugins, although apm marketplace add over SSH
succeeds. --ssh and --allow-protocol-fallback do not change this.

This looks like the github.com counterpart of #2063, which fixed the same scheme loss for non-GitHub hosts (there the
resolved dependency is now git: git@host:org/repo.git + path: and validates over SSH — confirmed on 0.33.0).

The fetch itself does honour SSH: when the API allowance is spent, apm logs "GitHub API rate limit hit ... skipping the
pre-flight accessibility probe and letting the download step confirm the package", and the same install then succeeds
over SSH. So only the pre-flight is wrong.

To Reproduce

1. A private github.com repo org/private-mkt with .claude-plugin/marketplace.json listing
   {"name": "alpha", "source": "./plugins/alpha"}, and SSH access to it.
2. With no GitHub token reachable by apm:

   apm marketplace add --name mkt ssh://git@github.com/org/private-mkt.git -r <sha>   # succeeds over SSH
   apm install -g -t claude alpha@mkt

3. Output: org/private-mkt/plugins/alpha#<sha> -- all probes failed (marker-file, Contents API, git ls-remote, shallow-fetch) -- verify the path and ref exist and that your credentials have read access; exit 1.

The same steps against a non-GitHub SSH host install fine.

Expected behavior

The pre-flight for an SSH-registered github.com marketplace's in-repo plugins probes over SSH (as validation does for
non-GitHub hosts since #2063), or is left to the download step, which already uses SSH.

Analysis (0.33.0)

• resolve_marketplace_plugin builds the right reference — DependencyReference(repo_url='org/private-mkt', host='github.com', explicit_scheme='ssh', ssh_user='git', virtual_path='plugins/alpha', is_virtual=True) — but its
  canonical string is the shorthand org/private-mkt/plugins/alpha#<sha>, which carries no scheme.
• _validate_package_exists (install/validation.py) sends every virtual package on a GitHub host to
  _validate_virtual_package, i.e. GitHubPackageDownloader probes (raw + Contents API + anonymous HTTPS
  ls-remote). explicit_scheme='ssh' is not consulted; only non-GitHub hosts take _validate_ado_git_package, which
  honours the SSH preference. The marketplace path also constructs that downloader without the CLI's protocol
  preference, so --ssh has no effect there.
• Related, for non-GitHub SSH hosts on 0.33.0: the blobless partial-clone cache (git/db_v1/<key>__p) is cloned
  from https://<host>/<org>/<repo> first; only after that fails does apm do the SSH shallow fetch. So every first
  fetch of an SSH dependency makes one HTTPS attempt against the host.

Environment

• OS: macOS 27 (arm64)
• APM: 0.33.0 (Homebrew formula)
• Python: 3.14.8