fix(agents): Improve git hygiene in adk-pr-triage skill

Update the adk-pr-triage skill instructions for local review to ensure the repository state is correct before squashing, and to prevent the loss of the original author's attribution.

- Added an explicit `git fetch origin main` step before PR checkout.
- Modified the squash instructions to capture and apply the original author via the `--author` flag during the squashed commit.

Change-Id: I646a6e8280e91033806f65d34799347d73b5e063