Add release provenance SBOM and installation verification

[Scriptception]

Problem

Pinned workflows and a lockfile protect development, but users cannot verify package provenance, inspect included dependencies, or confirm that an installed artifact matches an official release.

User outcome

Users can trace an installed Skilldeck package and bundled skills to an immutable source release.

Scope

• Generate build provenance and an SBOM for wheel and source artifacts.
• Publish checksums and verification instructions.
• Expose package and bundled-skill source metadata through a CLI command.
• Verify generated Claude plugin content against the same source tree.

Non-goals

• Creating a custom package-signing ecosystem.
• Collecting install information from users.

Acceptance criteria

• Every release publishes provenance, SBOM, and checksums tied to the tag commit.
• A clean environment can verify downloaded artifacts.
• The CLI reports package version, source reference, and bundled skill versions.
• Plugin and Python distributions contain identical canonical skill content.

Validation

• Verify artifacts after download from each supported release channel.
• Tamper with an artifact and confirm verification fails.

Relationships

• Runs with the first package release.
• Supports authenticated catalog and bundle features.

[Scriptception]

Implementation is in draft PR #81. It adds provenance-stamped artifacts, SPDX SBOM/checksum generation and verification, cross-distribution identity checks, attestations, and verifier hardening. Local validation passed: 156 tests, Ruff, formatting, mypy, plugin freshness, and diff check. First tag/PyPI publication and external attestation readback remain issue #80's gate.

[Scriptception]

Implementation status: PR #81 is out of draft and all repository CI checks are green. It adds provenance/SBOM/checksum generation, artifact verification, CLI source metadata, plugin identity checks, tamper tests, and release documentation.

Remaining acceptance evidence is external and intentionally still open: the repository has no published tag/release, the PyPI skilldeck endpoint is not yet published, and the configured PyPI trusted-publisher environment must be created before first-release readback can verify each release channel. That publication gate belongs with the first-release issue.