docs: CONTRIBUTING.md and prerequisites.md disagree on the reference agent runtime

[milamberspace]

What's broken

CONTRIBUTING.md and docs/prerequisites.md disagree on which agent runtime is the framework's reference:

• CONTRIBUTING.md → "The framework's reference runtime today is Claude Code", and its "Agent harnesses" table lists Claude Code as Primary, fully supported, Codex as Partial, and does not mention OpenCode at all.
• docs/prerequisites.md → "OpenCode is the reference implementation", Claude Code is "a second complete implementation", and Codex is "experimental".

Which layer

• CONTRIBUTING.md — the ## Agent harnesses section.
• docs/prerequisites.md — the "An agentic tool + access to an LLM" section.
• Canonical source: docs/vendor-neutrality.md lists the supported runtimes as "Claude Code; OpenCode; Codex adapter (experimental)".

How to reproduce

Read the "Agent harnesses" section of CONTRIBUTING.md, then the agentic-tool section of docs/prerequisites.md. They name different reference runtimes and give incompatible Codex/OpenCode states.

Expected vs actual

• Expected: the contributor guide and the prerequisites doc agree on the reference runtime(s) and each harness's support state.
• Actual: CONTRIBUTING.md still reflects the older "Claude Code is the sole reference runtime" framing (last touched 2026-05-26, docs(contributing): rewrite for framework-as-a-whole audience #323) and predates the vendor-neutrality work — OpenCode as reference implementation and the first-class Codex adapter both landed later (feat(codex): first-class Codex CLI skill runtime #902, 2026-07-26). CONTRIBUTING.md is the stale one; OpenCode appears in 9 other docs but never in it.

Surface area (optional)

Doc-only reconciliation of the CONTRIBUTING.md "Agent harnesses" section to match docs/prerequisites.md and docs/vendor-neutrality.md. No behaviour change.

Note: the two newer docs also differ slightly in emphasis — prerequisites.md calls OpenCode "the" reference implementation, while vendor-neutrality.md lists Claude Code and OpenCode side by side. The proposed fix uses the softer, co-reference framing (OpenCode and Claude Code both reference implementations, Codex experimental) to stay consistent with the canonical vendor-neutrality.md matrix.