#4312 docs(agents): prefer runner images and small fixes over selection code

Summary

On #4305, the agent kept the release job on the macOS 26 image and wrote an Xcode 27 selection step, although the provider's macOS 27 image has Xcode 27 as its default. Each bot finding then hardened the script instead of asking whether it should exist. #4311 removes it. This PR changes the guidance that let it happen:

• docs/agents/code-ownership.md, new "CI toolchains": select a preinstalled toolchain, such as Xcode, with the runner image in runs-on; if no image provides it, add selection code and record why. The AGENTS.md routing row now names CI.
• docs/agents/pull-requests.md: fix accepted findings with the smallest change, and prefer configuration or deletion to new code.
• To stay within the guidance byte budgets, it drops the two 1,000-line file rules that oxlint.config.ts max-lines already enforces, for source and test files.

4 files. The root cause and wording were cross-checked with an independent reviewer on another model provider. That reviewer found no other in-job toolchain selection in .github.

Validation

Head 171d56edef: pnpm check:affected --run --head 171d56edef passed, exactCommit=true, including agent-guidance: docs/agents 39,992/40,000 bytes, AGENTS.md 3,995/4,000. Docs-only, so runtime tests do not apply.

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