chore: remove unused CancellablePromise and gracefullyCancel utilities

[detail-app]

Removes the @internal CancellablePromise class and the gracefullyCancel
function from agents/src/utils.ts (~85 LOC). Neither symbol had any
production or test consumers anywhere in the monorepo — gracefullyCancel
was the only would-be consumer of CancellablePromise, and it itself had
zero callers.

Description

Removes two dead @internal symbols from agents/src/utils.ts:
CancellablePromise and gracefullyCancel. The API report
(agents/etc/agents.api.md) is updated to drop the corresponding entries.

Changes Made

• Removed the CancellablePromise class and gracefullyCancel function
  from agents/src/utils.ts (both marked @internal, so not part of the
  public API surface).
• Updated agents/etc/agents.api.md to remove the documentation entries for
  the removed symbols and fix one shifted diagnostic line reference.

Pre-Review Checklist

• Build passes: pnpm build, pnpm typecheck, and
  pnpm vitest run src/utils.test.ts (57/57) all pass.
• AI-generated code reviewed: Detail: Dead Code
• Changes explained: see below
• Scope appropriate: only the dead symbols and their API-report
  entries were removed.
• Video demo: n/a (no behavior change)

Testing

• Automated tests pass: pnpm vitest run src/utils.test.ts (57/57).
• restaurant/realtime agents: not run (internal deletion, no runtime
  consumers).

Additional Notes

Verified dead: a ripgrep across the entire monorepo found only
self-references within utils.ts for both symbols. The remaining dead
candidates in the file (isImmutableArray, ExpFilter.filtered,
ExpFilter.alpha setter, standalone waitFor) were each under 10 lines
and were left in place per the dead-code "tiny fragments" rule.

History

• Last materially changed September 2024 (Multimodal Agent with complete API #74 @bcherry)

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Dead Code PRs can be configured here.

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Latest commit: c80c52b

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.