Add pre-commit hook to run cargo fmt and clippy checks

[aisrael]

Summary

• Adds a tracked .githooks/pre-commit script that runs cargo fmt -- --check and cargo clippy --all-targets -- -D warnings before every commit, mirroring the checks already gated in CI (.github/workflows/ci.yml).
• Adds a short note in AGENTS.md explaining that new clones/worktrees need to run git config core.hooksPath .githooks once to activate the hook, since core.hooksPath is a per-clone git config and can't be committed.

Changes

• .githooks/pre-commit (new, executable) — runs fmt check then clippy, aborting the commit with a clear message if either fails.
• AGENTS.md — one-line note on enabling the hook via core.hooksPath.

Testing

• Ran the hook directly (./.githooks/pre-commit) to confirm it passes on a clean tree.
• Verified git config core.hooksPath is set to .githooks and that git commit invokes the hook.
• Confirmed the hook blocks a commit when a formatting violation is introduced, and allows a commit through when the tree is clean.
• Ran cargo fmt -- --check, cargo clippy --all-targets -- -D warnings, and cargo test on this branch — all pass.

Checklist

• Tests pass
• Documentation updated (AGENTS.md)
• No breaking changes

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