Release mode does not discover skills/* APM packages

Problem

APM documents skills/<name>/SKILL.md as a supported repository layout for a
collection of skills. Each skill can also be a hybrid package with its own
apm.yml.

microsoft/apm-action@v1 with mode: release currently detects an aggregator
only when it finds plugins/<name>/apm.yml. A repository containing
skills/<name>/apm.yml is classified as single-plugin, so release mode reads
and packs only the root apm.yml instead of discovering the individual skill
packages.

The relevant implementation is detectShape() and discoverPackages() in
src/release.ts.

Generic reproduction

collection/
├── apm.yml
└── skills/
    ├── alpha/
    │   ├── SKILL.md
    │   └── apm.yml
    └── beta/
        ├── SKILL.md
        └── apm.yml

Run the documented tag-triggered release workflow:

- uses: microsoft/apm-action@v1
  with:
    mode: release
    release-skip-publish: true

The action reports single-plugin and discovers the root package rather than
alpha and beta.

Expected behavior

Release mode should support APM's multi-skill collection layout, either by
discovering skills/*/apm.yml or through an explicit package-path or glob input
that avoids ambiguous shape inference.

Relevant APM package-layout documentation:
https://github.com/microsoft/apm/blob/main/packages/apm-guide/.apm/skills/apm-usage/package-authoring.md

Read detectShape() and discoverPackages() in src/release.ts, then reproduce the issue with the collection layout shown in the report and the documented release workflow. Verify how plugins//apm.yml is detected and how skills//apm.yml is classified. Done means release mode discovers alpha and beta packages instead of treating the repository as a single plugin.