[Security] Add cryptographically verifiable release artifacts

[Rwanbt]

Problem

Release integrity currently stops at SHA-256. ainative update verifies the
release archive digest before writing a byte, and docs/DISTRIBUTION-LIFECYCLE.md
section 11 states the boundary precisely: the digest detects corruption or a
substituted archive on the wire, but does not protect against a
compromised release source - whoever controls the release controls both
the archive and its digest.

Expected outcome

Cryptographically verifiable release artifacts, so a client can confirm a
release came from the maintainers, not just that it matches a digest the same
source published.

Acceptance criteria

• Investigation note comparing: Sigstore/cosign keyless signing, signed Git tags, and GitHub release attestations
• A chosen mechanism with its key-management story written down (who signs, from where, what recovery looks like)
• ainative update verifies the new signature and fails closed on mismatch, with a clear error distinct from UPDATE_INTEGRITY_FAILED
• A stated threat model delta: what attacks are covered after this change that were not covered before
• Signing is claimed in the docs only after it is actually enforced by the updater

Context

• Current, accurately stated boundary: docs/DISTRIBUTION-LIFECYCLE.md section 8 ("Integrity, stated precisely") and UPDATING.md
• The archive path/entry validation and size bounds are independent and stay

Out of scope

• Implying a current vulnerability: there is none inside the declared threat model - this is hardening against a stronger adversary
• Repackaging or changing the archive format