Add Longbridge agent

[huacnlee]

Summary

Adds Longbridge to the registry — an ACP agent for equity research and portfolio analysis on US and HK markets.

• longbridge/agent.json — binary distribution for darwin-aarch64, darwin-x86_64, linux-aarch64, linux-x86_64, windows-x86_64, each pinned with a SHA-256 checksum, launched via longbridge acp
• longbridge/icon.svg — 16x16 monochrome SVG using currentColor

Source: https://github.com/longbridge/longbridge-terminal (release v0.27.1)

Validation

$ uv run --with jsonschema .github/workflows/build_registry.py
Added agent: longbridge v0.27.1

$ python3 .github/workflows/verify_agents.py --auth-check --agent longbridge
[1/1] longbridge (binary)
    ✓ Success: Auth OK: longbridge-login(terminal)
  Passed: 1  Failed: 0  Skipped: 0

Auth is exposed as a terminal method (longbridge auth login, Longbridge OAuth), per AUTHENTICATION.md.

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