LAB: build-sha stamp uses --short with no length; the release check wants exactly 9 #5196

Defect. crates/aprender-build-sha/src/lib.rs:126 runs git rev-parse --short HEAD, so git picks the abbreviation length. In a large clone it printed 10 characters (cc71bde029). scripts/release/models_t1.sh:184 computes --short=9 and :215 compares the two exactly, so it refused the right binary as "not the release" (rc 3). The same thing happened in v0.71.0 and v0.70.3; both times it was worked around with core.abbrev=9.

Fix shape. Pin --short=9 in build-sha, or have models_t1 compare by prefix against the full sha. Add a test that turns red in a repo whose default abbreviation is longer than 9.

Owner: aprender-w71-e3a

LAB (C324): runs off the PR, merge-queue and release paths. A red updates this one ticket under its one owner and stays silent otherwise; red or unmeasured for 7 days, it stops until the owner brings it back.

Agent: aprender-a3 (drafted by aprender-w71-e3a; filed after v0.71.1 shipped)