feat(spec): resolve templates from a direct git url@ref

[w4bremer]

Summary

Adds a third way to resolve a solution target's template, alongside a local folder and a registry package: a direct git URL with a ref.

targets:
  - name: go-sdk
    inputs: [counter.idl]
    output: ./out
    template: https://github.com/me/template.git@v1.2.0
  - name: ssh-example
    template: git@github.com:me/template.git@main

The ref may be a tag, branch, or commit. The repository is cloned into the template cache, checked out at the ref, and used exactly like a registry template (it must contain templates/ and rules.yaml).

Resolution order

A template value is resolved in this order:

1. Local folder — <solution-dir>/<template> is a directory
2. Direct git URL — https://, http://, ssh://, git://, file://, or scp-style user@host:path, with the ref after the path's @
3. Registry package — e.g. apigear-io/template-cpp

Git URLs are detected before the registry lookup, so an scp URL (which contains two @) never reaches the registry repo-id parser. Local-folder and registry resolution are unchanged for all existing inputs.

Implementation

• repos.IsGitURL / SplitGitURLVersion / RepoNameFromGitURL — detect a git URL and derive a host-qualified cache key. The version split is anchored to the URL path, so scp user@host, HTTP credentials, and slash-bearing branch names all parse correctly. The https and scp forms of a repo map to the same cache entry.
• repos.GetOrInstallTemplateFromGitURL + cache.InstallRef — clone and check out a ref, reusing the cache when present. The existing registry path (Install / CheckoutTag) is untouched.
• git.CheckoutRef — resolves a ref as tag → local branch → origin/<ref> → commit hash, then checks out the resolved hash.
• spec.compute — routes git URLs to the new resolver before the registry.
• Schema: the template description documents the new form (no structural change).

Notes

• The cache is keyed by name@ref, consistent with the existing registry flow. For immutable refs (tags/commits) this is ideal; a branch ref is cached and not re-pulled until the cache entry is removed.

Testing

• Unit tests for URL detection, version splitting, and cache-key derivation.
• Integration tests (real clones from a local repo, offline) for ref checkout, install/caching, and the solution-target wiring.
• A regression test verifying scp-style URLs route to the git resolver rather than crashing the CLI.
• Full suite passes with -race; gofmt/go vet clean. Verified end-to-end through the CLI against a file:// template.