notebook-uniformity hook: drive it from the convention points

[classiqdor]

Follow-up to the conventions kit (#1673) and the pre-commit-tools refactor (#1686).

What

The notebook-uniformity pre-commit hook used to re-implement the same regexes that already live in .internal/conventions/points/. It now imports the points and applies their fix() instead — one source of truth per convention.

• The hook loads every point marked enforced, applies its fix(), then reports any enforced point that still fails (e.g. opens_h1, which has no auto-fix and can only be reported).
• Points are reached through a new points symlink → ../conventions/points (mode 120000), so the import is a plain from points._model import … with no sys.path juggling — same spirit as metadata_validate importing its siblings.
• Adding or changing an enforced rule is now a one-file edit under points/; the hook needs no changes.

Unchanged

• The enforced set is identical: show, result_value, references, single_h1, opens_h1.
• Behaviour: auto-fix on commit, fail so you re-git add, report the unfixable ones.
• Hook shrinks from ~200 to ~80 lines.

Verification

• Clean notebooks → no-op (ran on real notebooks with auto-fix on; zero false modifications).
• Crafted violations (qprog.show(), singular ## Reference) → auto-fixed to show(qprog) / ## References, reported, exit 1; re-run → clean.
• The validate symlinks hook passes on the new symlink.

Note

Depends on the points living on main (merged via #1673) — safe to merge now.