fix(tui): hide editor context from transcript

[kitlangton]

Summary

• admit Zed editor context as a separate descriptionless synthetic message with resume: false
• send the user-authored text unchanged as the following visible prompt
• document the project rule to get user buy-in before adding complexity for speculative, vanishingly unlikely races or security edge cases

Root cause

The V2 prompt migration flattened the synthetic editor-context part and user-authored text into one user message. The V2 renderer therefore displayed the internal <system-reminder> verbatim.

The existing synthetic-message projection already hides descriptionless synthetic context, so preserving that boundary fixes the transcript without adding display metadata or special rendering paths.

Verification

• bun typecheck in packages/tui
• bun run test in packages/tui (223 passed, 1 skipped)
• repository-wide push-hook typecheck (31 packages)