chore: add shared git hooks and CONTRIBUTING

[xinyandai]

What

Moves the local git hooks into a tracked .githooks/ directory so they can be shared across clones, and adds CONTRIBUTING.md.

• .githooks/pre-commit — forbids direct commits to main/master, runs cargo fmt --all --check, and runs cargo clippy --all-features --all-targets -- -D warnings.
• .githooks/pre-push — runs cargo test --all-features.
• CONTRIBUTING.md — documents the one-time git config core.hooksPath .githooks setup, what each hook does, the feature-branch/PR workflow, and the CI checks.

Why

Previously the hooks lived in .git/hooks/ (local, not shared on clone). Tracking them in .githooks/ lets every contributor opt in with a single core.hooksPath setting and keeps the local gate in lockstep with CI (fmt, clippy, tests).

Enabling

core.hooksPath is a per-clone local setting (not committed), so each clone runs once:

git config core.hooksPath .githooks

Verification

• Hooks are committed with the executable bit (100755).
• The pre-commit hook self-ran on this branch's commit (fmt + clippy) and passed.
• pre-commit was independently tested to block bad formatting and commits on main; pre-push runs the full suite (24 tests + doctest).

🤖 Generated with Claude Code

[xinyandai]

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