#247: one condition, one diagnosis — and the scratch repos stop being able to hang the suite

[Polichinel]

Closes #247. The last story of epic #241.

One condition was producing two diagnoses, neither of them the right one

A ref this clone cannot see and a ref that is not a frozen commit shared a message that named neither cause and never said git fetch. They have different remedies. Now three branches:

condition	diagnosis
the pin is blank	a defect in the pin — an empty ref reads the index
this clone has no such object	run git fetch — shallow, --single-branch, or older than the pin
it resolves to something that is not a commit	a defect in the pin

The bogus-sha case is deliberately classified as "this clone cannot see it", because that is the truth: the reader cannot tell a bad pin from a missing fetch, and the message now says so rather than guessing.

rows() refuses a scalar row by name

[test_environment] on the live registry is top-level strings, not sub-tables. Classifying such a table CONSUMED — which the partition check's own remediation message invites — used to return an AttributeError from a dict comprehension, in the module whose stated justification is failing legibly.

The scratch repositories are hermetic

All three now run git with -c commit.gpgsign=false -c core.hooksPath=/dev/null and an explicit timeout.

Verified rather than assumed: ran the suite under a HOME whose .gitconfig sets commit.gpgsign = true and points core.hooksPath at a nonexistent directory. Four tests pass where they would previously have failed with a bare CalledProcessError — or, with a passphrase-protected key, blocked on pinentry with no timeout and hung the whole run.

Mutation-proven three ways, each reverted.

C-91 resolved and moved; 22 open → 21.

ruff check .    # clean
pytest -q       # 421 passed, 1 skipped, 40 xfailed

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