Run make quality before every commit with a git pre-commit hook #4206

What

Run make quality before every commit, with a git pre-commit hook:

• A committed .githooks/pre-commit runs make quality from the root of the working tree.
• The commit fails when a check fails, or when ruff's fixes changed files that still need staging. The hook lists those files.
• It also fails, with a message saying what to do, when uv or make isn't installed or the worktree has no dev environment (.venv).
• make install turns the hook on with git config core.hooksPath .githooks. That setting is shared by every worktree of the clone, and each worktree runs its own checkout's hook.
• AGENTS.md, the GitHub PRs skill and README.md say to run make install once per clone.
• A registered rule with a proof checks the hook's behavior in a temporary repository.

Why

make quality (#4203) runs the same checks as pull-request CI, but only when someone remembers to run it. The hook runs it before an agent's or a person's commit, so problems show in seconds instead of after a CI run. This is tracker task 6.4 · 10, following decision 6.4 · 9: a git pre-commit hook running all of make quality, not a Claude Code hook. make quality takes about 3 seconds with warm caches and about 10 seconds cold. git commit --no-verify still skips the hook, and CI's Quality checks job remains the required check.