feat: add explicit emoji support to agent registration and updates

[bertheto]

Summary

• Enable callers to supply a custom agent emoji explicitly via REST, MCP, and Web UI registration/update flows
• Add �alidateEmoji() utility using Unicode Emoji_Presentation regex to prevent arbitrary strings from degrading avatar rendering
• Extend
  egisterAgent() and updateAgent() in MemoryStore to accept optional emoji, preserving the existing deterministic hash-based fallback
• Wire emoji input through REST (POST /register, PUT /:agentId), MCP (�gent_register, �gent_update, �us_connect), and the Web UI settings tab
• Add a Browser User Identity card in the UI settings pane allowing users to set their display name and avatar emoji, persisted in localStorage and synced via REST PUT

Test plan

• 5 unit tests in est_agent_registry.test.ts: explicit emoji persistence, fallback to generated, update semantics, preserve on unrelated update, validateEmoji edge cases
• 4 HTTP integration tests in est_agent_capabilities.test.ts: register with explicit emoji, register with invalid emoji (fallback), update emoji via PUT, list includes emoji
• 2 bus_connect integration tests in �usConnectIntegration.test.ts: emoji on new agent registration, emoji ignored on resume path
• All 51 targeted tests pass; full suite 593/598 (5 pre-existing failures in CLI sessions + config registry, unrelated)
• Manual Web UI smoke test: open Settings > UI tab, set display name + emoji, save, refresh page, verify avatar propagation in thread messages

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