Reproducible builds, code signing, and supply chain security

## Description

Per spec T136 and whitepaper supply chain requirements:

- Reproducible build configuration
- Code-signing verification (agent refuses dispatch to unattested/unsigned agents)
- Build provenance metadata (git commit, build timestamp) — partially in place via build.rs
- Independent audit readiness for Phase 3

## Requirements

- Deterministic compilation producing identical binaries from same source
- Ed25519 code signing for release binaries
- Agent version verification on heartbeat (reject unknown versions)
- Build provenance chain: source commit → CI build → signed artifact → distribution
- Signer ≠ approver enforcement for release artifacts

## Success Criteria

- [ ] Two independent builds from same commit produce identical binary
- [ ] Release binaries are Ed25519 signed
- [ ] Agent rejects dispatch from unsigned/unattested peers
- [ ] Build provenance metadata verifiable end-to-end
- [ ] Signer ≠ approver enforced for release artifacts
- [ ] Audit-ready documentation of supply chain

## Testing (Principle V)

- Build twice from same commit → verify identical output
- Sign binary → distribute → verify signature on recipient
- Deploy unsigned agent → verify cluster rejects it
- Attempt same-identity sign + approve → verify rejected

Start with build.rs, which already carries part of the provenance metadata, then trace the release, heartbeat, and dispatch entry points named in the requirements. Turn the success criteria into tests for deterministic builds, Ed25519 verification, unsigned-agent rejection, provenance checks, and signer/approver separation.

Written by the indexing model from the issue text.