* feat(provenance): add SLSA provenance and attestation signing libraries
Add internal/provenance for deriving in-toto subjects from Packer
artifacts, building SLSA Provenance v1 predicates, wrapping in-toto
statements, and best-effort git/CI source detection.
Add internal/attestation for DSSE envelope handling and a pluggable
Signer/Verifier backend supporting key (local PEM), kms (aws/gcp/
azure/hashivault), and keyless (Sigstore Fulcio) modes, plus Sigstore
bundle handling and DSSE/policy verification.
Add the supporting module dependencies in go.mod/go.sum.
* feat(provenance): add provenance post-processor
Add the opt-in "provenance" post-processor that runs after a build,
derives subjects from the artifact, emits DSSE-wrapped SLSA provenance
(and optional SBOM) attestations, signs them via the configured
signing backend, and writes sidecar files (including *.sigstore.json
bundles in keyless mode). Register it in the core post-processor set.
The provenance enable flag is a tri-state so an unset value stays
enabled through HCL2 decoding instead of being silently disabled.
* feat(provenance): add verify-attestation command
Add "packer verify-attestation" to verify signed DSSE attestations
against key, KMS, and keyless policy inputs, including optional
Sigstore bundle checks for Rekor and timestamp evidence. Register the
command in the CLI.
* docs(provenance): add reference CI workflows and changelog
Add reference GitHub Actions workflows under examples/ci for SLSA L2
keyless signing and L3-compatible delegated signing
* fix: lint and tests
* Added docs for Provenance PostProcessor
