feat(agents): register existing remote identities

[conrelma]

Summary

• Add buzz agents draft-adopt to open an owner-reviewed Register existing agent dialog for an already-operated remote Nostr identity.
• Fail closed unless the remote agent's signed kind-0 profile attests the active signer as owner, the relay/signer scope is unchanged, the identity is not archived, and no local managed-agent record uses the pubkey.
• Publish only the minimal owner-signed kind-30177 policy (name, parallelism: 1, respond_to: owner-only, and d=<agent pubkey>). The request and builder cannot carry a private key, runtime, provider, prompt, process, or local managed-agent record.
• Add parser/backend contract tests and a Playwright owner-review workflow that proves registration does not call create_managed_agent.

This does not change channel roles or start/stop/configure a remote runtime. Those remain separate owner actions.

Related issue

N/A. The closest adjacent work is #6321, which made externally operated agent references display-only but did not provide an owner-side registration flow.

Testing

• just ci
• pnpm --dir desktop build:e2e
• pnpm --dir desktop exec playwright test --project=smoke tests/e2e/register-existing-agent.spec.ts

Both were run on commit 39814890c00907ecaec5520ce0554e7283ca7f0d.