[RefImpl] Decide where shared host adapters live #313

Raised in the PR #256 review.

reference-implementations/README.md (PR #256) says host adapters are maintained apart from Guardians and that existing shims move to a shared place. It names no place, and two candidates exist:

• reference-implementations/agt/hosts/ and agt/packages/host-adapter/, inside the TypeScript workspace, where the host tests start the TypeScript Guardian (import { startGuardian } from "guardian").
• adapters/, at the top level, proposed by PR Add adapters/ with Claude Code, Cursor, and NAT reference implementations #22.

Ports also carry their own shims (PR #169 ships Codex and OpenCode ones). Issue #230 moves the wire signing code into a shared package, which a shared host location would depend on.

Decision needed: where shared host adapters live, and whether their tests run against any Guardian over HTTP rather than importing the TypeScript one. The README links here until this is decided.