docs: fix conflicting Copilot CLI custom agent precedence guidance

[BaconMan1168]

Why:

Closes: #46100

What's being changed (if available, include any code snippets, screenshots, or gifs):

The custom agent how-to and the CLI configuration directory reference stated opposite precedence rules for when a personal agent and a repository agent share the same name. I checked GitHub Copilot CLI's actual behavior directly (its skill-discovery mechanism, which follows the same personal-versus-repository resolution as agents) and confirmed that the repository-level definition takes precedence, and that matching is based on the name value in the frontmatter rather than the file name. Both pages now state this consistently.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/copilot-cli/customize-copilot/create-custom-agents-for-cli.md	fpt ghec	fpt ghec
copilot/reference/copilot-cli-reference/cli-config-dir-reference.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[subatoi]

I'm afraid this hasn't been approved for contributors as we've had no chance to investigate and figure out the best way to solve the problem in question, so I'm going to close this out. Sorry this won't be the outcome you were hoping for