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.
Tanmay Jain
d9ddc0bac8