[codex] Add Resend manifest policy tests

[mimeding]

Summary

Adds Resend manifest contract tests and tightens tool policies so sending, replying, reading a full thread, and labeling require approval while thread listing remains automatic.

Business Rationale

Email contains sensitive user and third-party content. This keeps high-risk actions behind approval while still allowing lightweight thread discovery that helps users and models orient before taking action.

Coding Rationale

The manifest already controls host-visible tool permissions. Encoding the intended permission split in tests prevents accidental regressions and keeps privacy-sensitive operations explicit.

Expected LLM Performance Improvement

Models should choose safer email workflows: list candidate threads first, request approval before reading or changing private content, and avoid unapproved sends or labels.

Changes

• Behavior change
• UI change
• Refactor / chore
• Tests
• Docs

Test Plan

• swift test --no-parallel
• swift build -c release
• git diff --check

Note: plain swift test currently exposes shared mock-host state across concurrent Swift Testing suites in this package. The serialized test run is green and exercises the new manifest contract.