chore: add shared git hooks and CONTRIBUTING

.githooks/pre-commit

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+#!/bin/sh
+# Shared pre-commit checks for the niche-value crate.
+# Enable with: git config core.hooksPath .githooks
+# Bypass the whole hook with `git commit --no-verify` if you must.
+
+# 1. Forbid direct commits to protected branches.
+protected="main master"
+branch="$(git symbolic-ref --short HEAD 2>/dev/null)"
+for b in $protected; do
+	if [ "$branch" = "$b" ]; then
+		echo "pre-commit: direct commits to '$branch' are forbidden." >&2
+		echo "  Create a feature branch:  git switch -c my-change" >&2
+		echo "  (bypass with 'git commit --no-verify' only if you must)" >&2
+		exit 1
+	fi
+done
+
+# Run cargo checks from the repo root (hook may be invoked from a subdir).
+root="$(git rev-parse --show-toplevel)" || exit 1
+cd "$root" || exit 1
+
+# 2. Formatting must be clean.
+echo "pre-commit: cargo fmt --check"
+if ! cargo fmt --all --check; then
+	echo "pre-commit: formatting issues found. Run 'cargo fmt --all', then re-stage." >&2
+	exit 1
+fi
+
+# 3. Clippy with warnings denied, across all features and targets.
+echo "pre-commit: cargo clippy"
+if ! cargo clippy --all-features --all-targets -- -D warnings; then
+	echo "pre-commit: clippy reported issues. Fix them, then commit again." >&2
+	exit 1
+fi
+
+exit 0

.githooks/pre-push

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+#!/bin/sh
+# Shared pre-push check for the niche-value crate: run the test suite before
+# anything reaches the remote / CI.
+# Enable with: git config core.hooksPath .githooks
+# Bypass with `git push --no-verify`.
+
+root="$(git rev-parse --show-toplevel)" || exit 1
+cd "$root" || exit 1
+
+echo "pre-push: cargo test --all-features"
+if ! cargo test --all-features; then
+	echo "pre-push: tests failed. Push aborted (bypass with 'git push --no-verify')." >&2
+	exit 1
+fi
+
+exit 0

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping improve `niche-value`!
+
+## One-time setup: git hooks
+
+This repo ships shared git hooks in [`.githooks/`](.githooks). Enable them once
+per clone:
+
+```sh
+git config core.hooksPath .githooks
+```
+
+(`core.hooksPath` is a local setting and is not shared automatically, so each
+clone must run this once.)
+
+### What the hooks do
+
+- **pre-commit**
+  - forbids direct commits to `main` / `master` (work on a feature branch);
+  - runs `cargo fmt --all --check`;
+  - runs `cargo clippy --all-features --all-targets -- -D warnings`.
+- **pre-push**
+  - runs `cargo test --all-features`.
+
+Bypass in a pinch with `git commit --no-verify` / `git push --no-verify`.
+
+## Workflow
+
+`main` is protected. Make changes on a feature branch and open a pull request:
+
+```sh
+git switch -c my-change
+# ... edit ...
+git commit -m "..."      # pre-commit runs fmt + clippy
+git push -u origin my-change   # pre-push runs the tests
+gh pr create
+```
+
+## Checks run in CI
+
+CI (`.github/workflows/ci.yml`) runs, and PRs must pass: tests (stable),
+`build` on the MSRV (1.83), `rustfmt`, `clippy` (warnings denied), Miri, and
+docs. Running the hooks locally keeps you ahead of CI.
+
+## Manual commands
+
+```sh
+cargo fmt --all
+cargo clippy --all-features --all-targets -- -D warnings
+cargo test --all-features
+cargo +nightly miri test --all-features   # requires: rustup +nightly component add miri
+```