docs: document local override precedence

[Whxuan0701]

Summary

• Document .apache-magpie-local/ as the personal, gitignored per-skill override layer and place it ahead of .apache-magpie-overrides/.
• Explain how the per-skill override chain relates to the existing <project-config>/project.md configuration-key chain.
• Record the additive-only guardrail for safety, confidentiality, privacy, and external-content-as-data requirements.

Validation

• uv run prek run --all-files
• git diff --check

All hooks passed, including doctoc, markdownlint, typos, lychee, placeholder checks, type checks, and tests.

Known limitations

• Documentation-only change; runtime behavior is unchanged.
• prek is not installed as a global executable in the local environment, so the repository check was invoked through uv run.

Closes #1003