Support pyproject harn package manifests

[xlr8harder]

Summary

Support harn package manifests declared in pyproject.toml under [tool.harn].

Why

The package docs describe [tool.harn] as a supported manifest location, but current resource discovery only reads package.json in the package-manager and extension-directory paths. That makes pyproject-only harn packages fail to expose declared extensions, skills, prompts, or themes.

Changes

• Prefer existing package.json manifests when present.
• Fall back to pyproject.toml [tool.harn] when no package.json manifest exists.
• Apply the same behavior to local extension directory entrypoint discovery.
• Add regression tests for pyproject-only package resources and pyproject-only extension directory manifests.

Testing

uv run --no-sync --all-packages pytest tests/test_coding_agent/test_package_manager.py tests/test_coding_agent/test_resources_and_extensions.py -q

Result: 26 passed