refactor: remove rhema-vercel-agent

The Vercel frontend existed to prove the shared harness extraction:
two frontends speaking the same wire protocol, one assembly, one
gate. The proof landed; the deployment didn't, and the crate cost a
full axum server, a Postgres store, and a Sandbox REST client to
keep carrying. It leaves the tree; git history keeps it for the day
a second frontend returns.

The Cloudflare DO is now the harness's one consumer, and nothing
downstream depended on the crate. README, AGENTS.md, and the two
dockerfiles lose their vercel entries; the cloud-agent and
cloudflare-code doc comments name the remaining frontend. SPEC.md
section 11 still tells the two-frontend story; it stays until the
direction settles.

deploy/docker/sandboxd.Dockerfile

@@ -42,7 +42,6 @@ COPY crates/rhema-sandbox-server/Cargo.toml crates/rhema-sandbox-server/Cargo.to
 COPY crates/rhema-sandbox-ssh/Cargo.toml crates/rhema-sandbox-ssh/Cargo.toml
 COPY crates/rhema-subagents/Cargo.toml crates/rhema-subagents/Cargo.toml
 COPY crates/rhema-transcript/Cargo.toml crates/rhema-transcript/Cargo.toml
-COPY crates/rhema-vercel-agent/Cargo.toml crates/rhema-vercel-agent/Cargo.toml
 COPY integrations/rhema-calendar-integration/Cargo.toml integrations/rhema-calendar-integration/Cargo.toml
 COPY integrations/rhema-forge-integration/Cargo.toml integrations/rhema-forge-integration/Cargo.toml
 # rhema-claw path-depends on the tools contract, which lives in its own