docs(agents): add PR scope and batching guidance

[pdmack]

Description

Agents opening one PR per file when multiple files share a root cause creates reviewer fatigue and clutters project history. AGENTS.md currently has no guidance on PR scope or batching.

Add a short "PR scope and batching" section explaining that changes with the same root cause belong in one PR, with a concrete example and a practical heuristic.

Definition of Done

• AGENTS.md has a "PR scope and batching" section under the existing Pull Requests guidance
• The guidance covers the root-cause grouping rule, an example, and a reviewer-facing heuristic

Resources

• Observed pattern: PRs docs(user): fix broken links in observability page #240, docs(dev): drop dead links to unwritten local-dev flow pages #241, docs(ngc-managed): fix broken links in cluster-management docs #242, docs(v0.6.0): fix broken links in observability snapshot page #261 each fixed one file but shared the same root cause (broken links from GitLab-to-GitHub migration) and could have been one PR