feat(desktop): register existing agents by pubkey

[DamirAGI]

Summary

• Add an owner-only Desktop flow to register an independently operated Nostr agent by its existing pubkey.
• Verify the live kind:0 NIP-OA owner attestation, reject locally managed identities, and publish only an owner-signed kind:30177 directory record.
• Make Desktop discovery policy explicit with respond_to plus an exact normalized allowlist; registration does not take key custody or create/start a local runtime.

Related issue

N/A. Closest existing work: #6571 implements a related owner-reviewed registration path but currently hard-codes owner-only discovery; #6321 covers display-only external agent references.

Testing

• just desktop-ci (passed on 3a582a1b89af59634c86022642ac241c083c2ba2; frontend 5882/5882, Tauri workspace tests passed)
• cd desktop && pnpm build:e2e && pnpm exec playwright test --project=integration tests/e2e/agents.spec.ts --grep "registers an existing agent without creating a local persona" (1/1 passed)
• Repo-wide just ci was also attempted. It reached the Rust test lane but was not green because of pre-existing crates/buzz-acp timing-test instability outside this diff; the affected Desktop package gate above completed successfully.

UI behavior is covered by the Playwright owner-review scenario; no production or installed Desktop state was modified.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 1c8321cd08feb597f8bcff5195c21148fb3e98ed...3a582a1b89af59634c86022642ac241c083c2ba2.
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