chore(agent): forbid Graphite workflows

[baxen]

Why

Buzz-managed agents currently receive general PR skills that may auto-select Graphite whenever gt is installed and the repository is Graphite-enabled. That adds an unnecessary workflow branch and can conflict with the desired standard Git behavior.

What

• Require Buzz-managed agents to use standard Git and the GitHub CLI for all branch, commit, push, and PR workflows.
• Explicitly forbid invoking gt or loading Graphite skills, even when other PR guidance prefers Graphite.
• Add a regression test that pins the compiled base-prompt contract.

Validation

• cargo test -p buzz-acp shared_base_prompt_forbids_graphite --lib -- --nocapture
• Normal pre-commit hook suite passed.
• Normal pre-push hook suite passed after removing duplicated user-level generic hooks; no hook bypass was used.

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