Bundle loader validates repo root instead of #subdirectory= path

Summary

When using #subdirectory= in bundle URLs, the loader validates the root of the cloned repository for bundle.md, pyproject.toml, or setup.py instead of validating the specified subdirectory.

Reproduction

Bundle include:

includes:
  - bundle: git+https://github.com/ramparte/amplifier-toolkit@main#subdirectory=bundles/deliberate-development

Error:

Clone missing expected files (pyproject.toml/setup.py/bundle.md): /home/user/.amplifier/cache/amplifier-toolkit-bb61039d65d7686a
Include failed (skipping): git+https://github.com/ramparte/amplifier-toolkit@main#subdirectory=bundles/deliberate-development - Clone of https://github.com/ramparte/amplifier-toolkit@main completed but result is invalid (missing pyproject.toml/setup.py/bundle.md).

Expected Behavior

The loader should validate the subdirectory path (bundles/deliberate-development/) for required files, not the repository root.

Actual Behavior

Repository structure:

amplifier-toolkit/           ← Loader validates HERE (no bundle.md)
├── README.md
├── bundles/
│   ├── deliberate-development/
│   │   └── bundle.md       ← Should validate HERE
│   └── m365-collab/
│       └── bundle.yaml

The clone succeeds, but validation fails because the root has no bundle.md/pyproject.toml/setup.py.

Impact

This breaks the monorepo pattern where multiple bundles live in subdirectories of a single repository. Users must create separate repos for each bundle as a workaround.

Suggested Fix

In the loader validation logic, when #subdirectory= is present in the URL:

1. Clone the repo (current behavior ✓)
2. Validate files exist in {clone_path}/{subdirectory}/ (not {clone_path}/)

Environment

• amplifier version: 2026.01.20-fafab09
• OS: WSL2 Ubuntu

Start by locating the bundle loader validation logic and tracing how the #subdirectory= value is parsed after cloning. Reproduce the provided amplifier-toolkit URL, then verify that required files are checked under bundles/deliberate-development/ and that the include loads successfully.