Supply chain: attest the Docker image and PyPI launcher (npm tarballs already carry provenance)

[debugmcpdev]

Current state

SUPPLY-CHAIN-SECURITY.md ("Verifying a Release") is explicit about the gap: what is attested today is the npm package tarballs — npm publishes run with --provenance, and release tarballs get GitHub build-provenance attestations plus SBOMs. The Docker image and the PyPI launcher are built by the same tag-triggered workflow but carry no attestations of their own:

• docker-publish pushes via docker/build-push-action with no provenance/SBOM output and no image-digest attestation.
• pypi-publish uploads with twine + token (permissions: contents: read — no id-token), so no PyPI trusted publishing and no PEP 740 attestations.

Proposal

1. Docker: attest the pushed image digest with actions/attest-build-provenance (subject index.docker.io/debugmcp/mcp-debugger@sha256:...), and/or enable BuildKit provenance + SBOM generation on docker/build-push-action. Verification story: gh attestation verify oci://docker.io/debugmcp/mcp-debugger:<tag> --repo debugmcp/mcp-debugger.
2. PyPI: migrate the launcher to PyPI trusted publishing via pypa/gh-action-pypi-publish (which generates and uploads PEP 740 attestations automatically) and drop the token — mirroring the npm OIDC setup already in place.
3. Update the "Verifying a Release" section of SUPPLY-CHAIN-SECURITY.md with the new verification commands and remove the "do not yet carry their own attestations" caveat.

This closes the remaining gap in the release-signing story (OpenSSF Best Practices signed_releases) — every distributed artifact, not just the npm tarballs, becomes independently verifiable back to the tagged commit and workflow.

[debugmcpdev]

Landed in #430 (squash 0e73344). Workflow + docs are in place, but the attestations themselves only prove out at the next tag push. Two actions remain before/at v0.25.0:

1. Before tagging: add the PyPI trusted publisher for debug-mcp-server-launcher (pypi.org → project Settings → Publishing → GitHub: owner debugmcp, repo mcp-debugger, workflow release.yml, environment blank). Without it the PyPI publish fails with invalid-publisher.
2. At release verification: gh attestation verify oci://index.docker.io/debugmcp/mcp-debugger:<tag> --repo debugmcp/mcp-debugger and confirm PEP 740 "Verified details" on the PyPI page; then delete the PYPI_TOKEN secret after the first successful OIDC publish.