ci: point Release Please at the core package version file

[Copilot]

The automated release workflow was reading the workspace root pyproject.toml, which is not the publishable Python package metadata source in this repo. As a result, Release Please could not resolve a valid package version and failed while validating release inputs.

• Release workflow

  • Update Release Please to read version metadata from packages/core/pyproject.toml, which contains the imednet package version tracked for releases.
  • Keep package-name: imednet unchanged so workflow metadata stays aligned with the core package.

• Contributor docs

  • Clarify that automated release PRs update packages/core/pyproject.toml, not the workspace root pyproject.toml.
  • Align release workflow documentation with the repo’s workspace/package split to reduce future misconfiguration.

• Effective config change

  with:
    release-type: python
    package-name: imednet
    version-file: packages/core/pyproject.toml