feat(desktop): support authorized external agent identities

[zeolenon]

Summary

• allow relay-native agents to appear in mention autocomplete when they are
  current channel agents, present in the agent directory, and invocable by the
  current viewer;
• list invocable relay-native agents in a separate Connected agents section,
  with profile, messaging, channels, memories, and activity access;
• let an owner securely link an external agent's nsec in the OS keychain
  after verifying both key identity and NIP-OA ownership;
• allow linked identities to update their public display name, photo, and
  description without adopting or launching their external runtime;
• allow authorized relay-native agents to be selected and added from the
  channel member picker;
• preserve the existing behavior for locally managed agents;
• keep relay-only non-members and profile-only agents without a directory entry
  hidden;
• avoid attempting to start an external agent as a local managed runtime.

Why

Hermes' native Buzz gateway creates a relay-native agent identity rather than a
Buzz Desktop managed runtime. The current autocomplete filter only admits
locally managed agent identities, so a working Hermes agent can receive messages
but cannot be selected through @ autocomplete or found on the Agents screen.

Requiring channel membership, an agent-directory record, and the relay response
policy keeps the visibility boundary narrow and avoids exposing arbitrary relay
profiles.

External runtimes also need a safe, limited profile-management path. Treating
them as managed agents would incorrectly give Buzz lifecycle ownership and
could start a duplicate harness. The new link flow therefore stores only the
signing key in the existing OS-backed secret store and limits its surface to
public kind-0 profile fields.

Tests

• full desktop unit suite after the connected-agent upgrade: 3,782 passed;
• TypeScript typecheck;
• Biome checks on changed files;
• Rust unit tests for namespaced secret storage and non-echoing validation;
• E2E: relay-only allowlisted non-member remains hidden;
• E2E: directory-backed external channel agent is mentionable and does not start
  a local runtime;
• E2E: stopped local managed agent still starts when mentioned;
• E2E: invocable relay-native agents appear separately from local runtimes;
• E2E: link identity, edit public profile, and refresh the connected-agent
  card;
• E2E: add an authorized external agent through the channel member picker
  without creating or starting a local runtime.

Related: #2349, #3277.