fix(docs): broken link in AGENTS.md and CLAUDE.md (#34852)

Update broken contributing links in AGENTS.md and CLAUDE.md

Description: 

Update internal references from .github/CONTRIBUTING.md to [Contributing
Guide] to fix navigation issues for local contributors.

Proposed Changes

AGENTS.md: Change [.github/CONTRIBUTING.md] link text to [Contributing
Guide] since the file path is not present in the local root.

CLAUDE.md: Change [.github/CONTRIBUTING.md] link text to [Contributing
Guide] for consistency.

Reasoning: 

Users following these docs locally often find the specific file path
.github/CONTRIBUTING.md confusing or "broken" in markdown previews that
don't resolve the .github hidden directory correctly. Using the
descriptive label "Contributing Guide" is more user-friendly and
standard across the repo.

Checklist:

[x] Run make format, make lint and make test (N/A for these doc-only
changes).

[x] PR title follows the format: TYPE(SCOPE): DESCRIPTION.

[x] I have read the [Contributing
Guide](https://www.google.com/search?q=https://github.com/langchain-ai/langchain/blob/master/.github/CONTRIBUTING.md).