Phase 5: `sindri registry serve --sign-with` re-signing (ADR-028)

[pacphi]

Background

sindri registry serve ships in Phase 3 (#259) as a read-only embedded OCI registry over a components directory. The original spec called for an optional --sign-with <key> flag so operators could test end-to-end strict-OCI verification against a local dev registry.

In PR #259 the flag was stripped because shipping a flag that parses but no-ops would have been misleading. The use case (local signed-pull testing) is real but niche, and re-signing is non-trivial — it requires v4's first signing operation (everything else is verification).

In scope

• New CosignSigner sibling to the existing CosignVerifier (v4/crates/sindri-registry/src/signing.rs). Reads a cosign private key, signs a manifest digest per the cosign manifest spec.
• Wire --sign-with <keyfile> back onto sindri registry serve (v4/crates/sindri/src/commands/registry/serve.rs).
• When --sign-with is set, every served manifest is signed and the cosign signature manifest is injected into the served index.json per the spec.
• Optional --key-password-env <var> for password-protected keys; never read from prompt in a server (would block).
• Update v4/docs/SOURCES.md to remove the "Phase 5 follow-up" note on serve and document the flag.
• Tests: pull from serve --sign-with <key> via oci-distribution's client, verify the cosign signature against the configured key.

Out of scope

• Production signing infrastructure (KMS, HSM integration). --sign-with is for local dev/test of the trust chain.
• Re-signing in prefetch — that workflow's contract is verbatim manifest streaming (preserves upstream signatures); see Phase 3 follow-up rationale.

Verification

• sindri registry serve --root ./components --sign-with ./test-cosign.key is reachable from a LocalOciSource with supports_strict_oci() returning true against the configured key's trust scope.
• An apply --strict-oci against a registry served with --sign-with succeeds end-to-end with no network.
• Without --sign-with, served manifests are unsigned (current behaviour); --strict-oci correctly rejects them.

Reference

• ADR-028 — v4/docs/ADRs/028-component-source-modes.md
• DDD-08 — v4/docs/DDDs/08-registry-source-domain.md
• SOURCES.md — v4/docs/SOURCES.md
• Plan §5 — v4/docs/plan/source-modes-implementation.md
• ADR-014 (signed registries with cosign) — v4/docs/ADRs/014-signed-registries-cosign.md
• Existing CosignVerifier to mirror — v4/crates/sindri-registry/src/signing.rs
• Existing serve command — v4/crates/sindri/src/commands/registry/serve.rs
• Phase 3 follow-up that stripped the no-op flag — PR feat(v4): Phase 3 — GitSource, registry serve/prefetch, OCI blob streaming (ADR-028) #259