docs(claude): add Promise.race handler-stacking rule

[John-David Dalton (jdalton)]

Summary

• Adds a concise rule under SHARED STANDARDS documenting the Promise.race / Promise.any handler-stacking anti-pattern described in nodejs/node#17469 and @watchable/unpromise.
• Preventive, not reactive — no current socket-cli code exhibits the pattern. Prompted by fixes in sibling repos where the same pattern had crept in: socket-sdk-js#600 and socket-lib#184.

Test plan

• Docs-only change — no test plan required.

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Note

Low Risk
Docs-only change in CLAUDE.md; no runtime or behavioral impact beyond updated engineering guidance.

Overview
Adds a new SHARED STANDARDS guideline in CLAUDE.md warning against using Promise.race/Promise.any with long-lived promises inside loops due to accumulating .then handlers, and recommends safer concurrency-limiter signaling patterns (with references to nodejs/node#17469 and @watchable/unpromise).

^{Reviewed by Cursor Bugbot for commit c89ed2e. Configure here.}