feat(hex): add Hex MCP Plugin support

[cstavitsky]

Testing

• Tested locally via ngrok tunnel
• ran core suite locally

Summary

• Adds @sentry/junior-hex, a standalone plugin that connects Junior to Hex's hosted MCP server (create_thread / get_thread) via per-user OAuth — no shared API token needed.
• Adds guardrails for data integrity (no fabrication, always report data_as_of and gaps), query safety (read-only, scoped to org slug + time window), auth error handling (no retry on 401/403), and rate limits (cap of 6 create_thread / 30 get_thread calls per invocation).
• Wires the package into the release pipeline (.craft.yml, CI, bump script, CONTRIBUTING.md, README.md).
• Adds plugin docs at extend/hex-plugin.md with install, config, region table, verification steps, and failure modes.

[vercel]

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

There are 3 total unresolved issues (including 1 from previous review).

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[dcramer]

lgtm