Clarify CUA harness usage

[rgarcia]

Summary

• clarify when to use direct Agent control versus the harness shape
• note that CuaHarness is currently a thin wrapper that installs CUA defaults and adds transcript access

Tests

• Not run (docs-only change).

---

Note

Low Risk
Docs-only changes that clarify API guidance; no runtime behavior or data/security-sensitive code is modified.

Overview
Clarifies README guidance on when to use CuaAgent vs the harness shape by spelling out the kinds of control/features each implies (raw agent control vs app-layer helpers and higher-level queue/events).

Explicitly notes that CuaHarness is currently a thin wrapper that installs CUA defaults, delegates runtime methods to the wrapped Agent, and adds getTranscript().

^{Reviewed by Cursor Bugbot for commit 26b3d13. Bugbot is set up for automated code reviews on this repo. Configure here.}

[firetiger-agent]

Firetiger deploy monitoring skipped

This PR didn't match the auto-monitor filter configured on your GitHub connection:

Any PR that changes the kernel API. Monitor changes to API endpoints (packages/api/cmd/api/) and Temporal workflows (packages/api/lib/temporal) in the kernel repo

Reason: This is a docs-only change clarifying CUA harness usage, with no modifications to API endpoints or Temporal workflows.

To monitor this PR anyway, reply with @firetiger monitor this.