feat: add Reasonix agent to registry

[innocarpe]

Summary

Add the official Reasonix entry to the ACP registry, as requested in esengine/DeepSeek-Reasonix#3540.

• reasonix/agent.json uses 1.39.2, the current npm latest, for both the registry version and reasonix@1.39.2 --acp distribution.
• Add the required license_url for the MIT license under the current registry schema.
• Keep the existing 16x16 monochrome icon.
• The ACP initialize response advertises terminal auth (reasonix-setup, args setup), as implemented in Fix ACP Zed integration gaps / 修复 ACP 的 Zed 集成缺口 esengine/DeepSeek-Reasonix#4624.

Verification

• SKIP_URL_VALIDATION=1 uv run --with jsonschema .github/workflows/build_registry.py --dry-run — validated all 43 entries against the current schema. URL checks for the full registry were skipped; the Reasonix 1.39.2 npm tarball and MIT license URL were checked separately and both returned HTTP 200.
• npm view reasonix version bin dist.tarball — confirms version 1.39.2 and the reasonix executable.
• python3 -m json.tool reasonix/agent.json and git diff --check passed.

This supersedes the stale third-party entry attempt in #384 (version 1.0.0, unpinned package).