skills: teach skills about LIR schema and scalar function registry

[mgree]

Motivation

Teach skills about the new registries.

#38868
#37814

Description

New registries place new demands on the code we build; we need to have the various agents know about them.

Verification

haha 😅

[def-]

QA LLM Review

1. MEDIUM -- func_registry regenerate-before-bump silently clobbers the shipped version's digest

.agents/skills/mz-commit/SKILL.md:59

The first bullet makes REWRITE=1 cargo test -p mz-compute-types --test func_registry the immediate response to any registry failure, including "removed, or has a changed property"; the bump-first requirement only shows up two bullets later. A rewrite run before the bump overwrites the shipped version's entry in func_registry_digests.json with the new properties' digest, and nothing fails afterwards, so the record of what the shipped LIR version's functions meant is lost.

Details

Mechanism: the REWRITE branch in src/compute-types/tests/func_registry.rs:228 does digests.insert(version, digest(&properties)) keyed on the current LIR_VERSION, so a pre-bump rewrite replaces "1" rather than adding "2". Bumping afterwards and rewriting again leaves {"1": <new>, "2": <new>}. The checking path only asserts digests.get(&version) for the current version, so the corrupted entry is never revisited. That entry is the only key back into git history for the old version's properties (func_registry.json is rewritten in place, unlike lir_v{N}.json), which is exactly what the digests file exists for.

The test's own failure message orders it the other way round: bump, "so ... version N's digest in ... stays as it is", then regenerate. The LIR section of this same skill gets the ordering right at line 43. .agents/skills/mz-test/SKILL.md:36 inherits the same inverted order.

Fix: put the version decision ahead of the command in bullet 1, e.g. "If the failure names removed or changed functions, settle the LIR_VERSION question below before running REWRITE. A rewrite at the old version overwrites that version's digest entry instead of adding a new one, and no later check catches it."