box: a status git cannot read stops the call before the version commit is built

[Burntgogi]

Fixes #365.

A failed git status was discarded by the -z test, so a corrupt index read as clean and REMOTE_CMD exported BLOOMERY_GIT_COMMIT pointing at a commit whose dirt nobody saw.

• The status is a checked assignment now (st=$(git -C "$HERE" status --porcelain 2>/dev/null)), under the condition the check serves: a tree whose HEAD reads. A tree the status fails on stops rc 70 naming the tree and Git's exit code, before REMOTE_CMD is constructed. A tree with no readable HEAD keeps its previous path (COMMIT=unknown), which is not the defect the issue names.
• The self-test's scratch tree is a git repo with a committed HEAD (identity and hooks pinned), and a corrupt-index check sits at the boundary: printf 'x' > .git/index, then the call must end rc 70 naming $HERE and git exit 128, say what the mark would have been, and never reach ssh.

Checks on the fixed tree: tools/box.sh --self-test passes whole. The negative control — the same self-test against a copy carrying the old -z line — ends self-test 4 failed (rc 0, want 70, no tree/message in the output, and the command reached ssh).

[github-actions]

Verdict: CLEAN: The status check rejects Git failures before constructing the version export, and the regression assertion rejects the old behavior.

Non-blocking

⚠ Use English for the new comment: tools/box.sh:1144. CONTRIBUTING.md requires English comments. Replace the three lines with # Reject unreadable dirt before exporting a known commit; trees without HEAD keep unknown. This keeps the new contract readable without a later translation edit.

Checked

Duplicates: no new functions, types or constants; no shared owner needed for this short check.
Complexity: no Rust functions changed. Dryer and crapper did not run.
Silent failure: 1 changed status read followed through the version export. Failed reads, including partial stdout, exit 70. Direct, local and queued execution share the check.
Tests: bash syntax and five isolated guard cases passed. The original status line returns 0 on Git exit 128; the added want 70 assertion rejects that result. The PR body reports the full self-test and negative control.
Gates: one corrupt-index regression case added; none removed or relaxed. No pins moved.
Escape catalog: no new C1–C4 pattern matched; Git's status is captured before another command can replace it.

Notes

The full self-test was not rerun; it creates scratch files outside the permitted review outputs.

_{🔨 Shingler, bloomery's review bot: approves a clean pull request and requests changes on a must-fix (how it works)}

Filed outside this PR: #381, #382

[juliankang4]

@Burntgogi Thanks for the contribution. The review is done, so I'm merging this.