Add the .apache-magpie-local override layer to the AGENTS.md resolution order

[justinmclean]

Summary

AGENTS.md documents the configuration resolution order but omits .apache-magpie-local/,
the highest-precedence layer. The string does not appear in the file at all, while 59 skill files
already consult that directory. Add it.

Background

.apache-magpie-local/ is the personal, gitignored override layer: a per-person sibling to the
committed .apache-magpie-overrides/. It is read at runtime, it may add a person's paths,
wording or capability enablement, and it cannot weaken the safety, confidentiality or privacy
baseline. PR #841 documented it in tools/spec-loop/specs/adoption-and-setup.md with the
precedence:

.apache-magpie-local/ -> .apache-magpie-overrides/ -> organization defaults -> framework default

AGENTS.md § Configuration resolution order still shows a chain starting at
<project-config>/project.md with no mention of the local layer. Since AGENTS.md is the file
agents load first and presents itself as authoritative on where config values come from, an agent
reasoning about a value's origin does not know the top layer exists.

Note the two chains describe slightly different things — the AGENTS.md one is about
placeholder and dotted-key resolution through project.md and organization manifests, the spec
one is about per-skill override files. Making that relationship clear is part of the job; a bare
insertion into the existing code block may not be the clearest fix, and a short paragraph plus a
pointer to the spec section is equally acceptable.

Where to look

• AGENTS.md § Configuration resolution order — the fenced text block showing the chain,
  and the paragraphs around it.
• tools/spec-loop/specs/adoption-and-setup.md — search for apache-magpie-local; this is the
  authoritative description to summarise, including the additive-only guardrail.
• skills/pr-management-code-review/SKILL.md § Adopter overrides — a live example of a skill
  consulting both layers in the documented order.

Acceptance criteria

• AGENTS.md names .apache-magpie-local/ and states that it takes precedence over
  .apache-magpie-overrides/.
• It states the additive-only constraint (a local override cannot weaken the safety,
  confidentiality or privacy baseline).
• The relationship between the per-skill override chain and the existing config-key chain is
  clear to a reader who knows neither.
• prek run --all-files passes, including the lychee link check if you add links.

Estimated effort

~1 hour for someone new to the codebase.

Getting started

[How to contribute](https://github.com/apache/airflow-steward/blob/main/CONTRIBUTING.md#your-first-contribution)

This issue was drafted with the help of an AI-assisted tool and reviewed by a Magpie maintainer before posting. If anything here is unclear or looks wrong, say so on the issue: a real person is reading.