docs: reframe README opener around the human+agent audience

[constk]

Problem

The current README opens by describing what the harness is (gates, scaffold, controls) but not who it's for. A newcomer landing from a GitHub link has to infer the audience from the feature list, which weakens the pitch for the project's distinguishing angle: production-grade quality when humans and LLM agents share the keyboard.

Proposed solution

Add a two-line opener at the top of README.md, above the existing pitch:

This template is for teams pairing AI agents with human engineers. The harness keeps quality consistent regardless of who's at the keyboard.

No structural changes to the rest of the README required.

Acceptance criteria

• README.md leads with the human+agent audience framing
• Existing pitch and feature list remain intact below the new opener
• Wording is consistent with the repo description: "Production-grade SDLC scaffold for human–LLM coding collaborations"

Priority rationale

First impression for anyone discovering the repo; cheap to land and disproportionately changes how the harness reads.

[constk]

Closed via #99 (merged to develop as commit 59ad7f0). Awaiting the next release: PR to reach main.