Generate release note additions

[tconley1428]

What changed

• Rename the prepended changelog heading in release notes to "Notable Changes".
• Generate an SDK Core section at release time by comparing the sdk-core submodule gitlink at the previous Python SDK release tag with the release SHA.
• Link SDK Core commits and rewrite sdk-rust PR references to full cross-repo links.
• Fetch tags and submodules in release jobs that need release-note generation.

Validation

• python3 .github/scripts/release_verify.py changelog-notes --version 1.29.0 --output /tmp/sdk-python-release-notes-check.md
• uv run ruff check .github/scripts/release_verify.py
• uv run ruff format --check .github/scripts/release_verify.py
• python3 -m py_compile .github/scripts/release_verify.py
• git diff --check