Add paper: Harness Engineering for Language Agents (CAR) to 3.3 Code Agents

Per the "How to Contribute" guidance (create an issue for missing papers), suggesting an addition.

**Section:** 3.3 Code Agents (next number, 123)

**Entry:**
123. **Harness Engineering**: "Harness Engineering for Language Agents: The Harness Layer as Control, Agency, and Runtime" [2026-03] [[paper](https://www.preprints.org/manuscript/202603.1756/v2)]

**Why it fits:** It formalizes the **agent harness layer** — the runtime/control loop wrapping tool execution that powers coding agents (SWE-agent, OpenHands, etc.) — as **Control, Agency, and Runtime (CAR)**, audits 63 harness-relevant works, and proposes HarnessCard for reporting. Relevant to how code agents are built and evaluated.

Disclosure: I am an author. Happy to open a PR instead if you prefer. (DOI: 10.20944/preprints202603.1756)

In the repository's curated list, locate section 3.3 Code Agents and the next numbered entry. Add the supplied Harness Engineering paper entry as item 123, preserving the surrounding formatting and link style, then verify the numbering and paper URL.

Written by the indexing model from the issue text.