fix(hooks): check the whole codebase as staged in pre-commit

[paustint]

Summary

• The pre-commit hook now checks a snapshot of the index across the whole codebase. It used to pass staged paths to oxfmt and oxlint, which then read the working tree, so a file formatted on disk but never re-staged could slip through and unstaged edits could block a clean commit. commit -a and commit <path> are covered too, because git points GIT_INDEX_FILE at the right index.
• The hook header now names all four repos that share it, so every copy is byte-identical. This mirrors chore: adopt oxfmt and oxlint with a pre-commit hook simple-xml#19.
• checkout-index runs with --ignore-skip-worktree-bits, so files hidden by a sparse checkout or git update-index --skip-worktree are checked too. simple-xml gets the same change in fix(hooks): export skip-worktree entries into the pre-commit snapshot simple-xml#20, which keeps all four copies byte-identical.
• AGENTS.md describes the new behavior.

No CHANGELOG entry, since nothing under src/ changed.

Test plan

• Hook passes on a clean index, matching npm run format:check and npm run lint
• Hook blocks an unformatted file staged under src/
• Hook blocks a staged, unformatted file marked skip-worktree and removed from disk
• Hook removes its temporary snapshot afterwards

[Copilot]

🟡 Changes recommended

Sparse-checkout entries are omitted from the snapshot, undermining whole-index validation.

1 open finding

• Skip-worktree paths are omitted from index checks · New

What changed in this PR

Updates pre-commit validation to inspect a temporary snapshot of the Git index rather than the working tree.

Changes:

• Exports the index before running formatting and lint checks.
• Documents the snapshot-based hook behavior and shared repositories.

File	Description
.githooks/​pre-commit	Checks formatting and linting against an index snapshot.
AGENTS.md	Documents the revised hook behavior.

🧠 Review effort: Balanced

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[Copilot]

🟡 Changes recommended

The hook diverges from the repositories where it must remain byte-identical.

1 open finding

• Keep shared hooks byte-identical across repositories · New

1 resolved since last review

• Skip-worktree paths are omitted from index checks

🧠 Review effort: Balanced

[Copilot]

🟢 Approval recommended

The snapshot handling and documentation match the intended behavior, with no unresolved findings.

0 open findings

1 resolved since last review

• Keep shared hooks byte-identical across repositories

🧠 Review effort: Balanced