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]

🟢 Approval recommended

The focused hook and documentation changes correctly implement the stated staged-snapshot behavior.

0 open findings

What changed in this PR

Updates the pre-commit hook to validate the exact staged repository snapshot.

Changes:

• Checks formatting and linting across the staged codebase.
• Preserves unstaged working-tree changes.
• Documents the revised hook behavior.

File	Description
.githooks/​pre-commit	Creates, checks, and cleans up an index snapshot.
AGENTS.md	Documents whole-codebase staged validation.

🧠 Review effort: Balanced

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[Copilot]

🔵 Needs a closer look

A relative TMPDIR prevents temporary snapshot cleanup.

0 open findings

Previously missed (1)

In code that hasn't changed since last review

Relative TMPDIR causes snapshot cleanup to fail

.githooks/​pre-commit:31

If TMPDIR is relative, mktemp returns a relative path. After the hook changes into that directory, the EXIT trap resolves the path relative to the snapshot itself and fails to remove it, leaving a full source snapshot behind. Normalize the path before installing the trap.

🧠 Review effort: Balanced

[paustint]

Went through Copilot's previously-missed comment and it was valid:

• .githooks/pre-commit:31 — fixed (lands with the next push). A relative TMPDIR gave a relative snapshot path that the EXIT trap resolved from inside the snapshot after the cd, leaking the whole snapshot. The path is now anchored to the hook's working directory before the trap is installed. Verified cleanup with relative, absolute and unset TMPDIR.

[Copilot]

🟢 Approval recommended

The snapshot handling, cleanup, and documentation are consistent, with no unresolved findings.

0 open findings

🧠 Review effort: Balanced