Fix ADR AGENTS.md docs link

[JSap0914]

Fixes Fallout-build/docs.fallout.build#9

Summary:

• Replace the ADR-0007 AGENTS.md relative link that the docs.fallout.build deploy reports as broken.
• Point readers to the source repository's root AGENTS.md on GitHub, matching the existing docs pattern for root-level files.

Validation:

• Confirmed docs.fallout.build issue chore(ci): drop non-GitHub CI providers #9 reports ../../AGENTS.md from the Docusaurus deploy.
• Confirmed docusaurus.config.ts builds docs from ./fallout-source/docs.
• Confirmed docs/branching-and-release.md, docs/agents/release-and-versioning.md, and root AGENTS.md exist in source.
• curl -L for https://github.com/Fallout-build/Fallout/blob/main/AGENTS.md returned HTTP 200.
• git diff --check passed.

Note: I did not run a full Docusaurus build locally because npm ci in the docs-site clone did not complete promptly on this Windows-mounted worktree, so it was not a cheap validation path.

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[ITaluone]

Thanks for fixing this.