chore(hooks): mirror CI checks in pre-push hook

[BryanFRD]

Summary

Replaces the test-only pre-push with the fmt+clippy pair that gates the CI 'Test' job and currently slip through to GitHub Actions every push. The full test suite (cargo nextest) is intentionally left out of pre-push — the cargo crate is heavy enough that running tests on every push would push the hook well past the 3-minute developer-budget.

.githooks/pre-push runs:

• cargo fmt --all -- --check (mirrors test.Format check)
• cargo clippy --all-targets -- -D warnings (mirrors test.Clippy)

Skips:

• cargo nextest run --no-fail-fast — too slow for a pre-push budget; CI keeps running it.
• cargo audit / cargo deny / cargo machete — security job covers these in CI.
• Coverage / fixtures / benchmarks — CI artefacts, not type-soundness gates.

To activate: run bash .githooks/install.sh once (sets core.hooksPath=.githooks).

Test plan

• cargo fmt --check and cargo clippy --all-targets -- -D warnings pass on a clean checkout
• bash .githooks/install.sh then git push --dry-run does not crash

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