tech-debt: remove legacy field-shape fallbacks in altimate-core-* tools

Start by reviewing the #1090 changes in the eight listed packages/opencode/src/altimate/tools/altimate-core-*.ts files and locate each legacy fallback. Then inspect how each consumer receives its engine response and define the shape-drift test for that consumer. Done means the fallbacks are removed, missing primary fields fail loudly, and CI catches absent fields in real engine responses.

Deferred from the v0.9.6 release review (Tech Lead persona, P1).

Context

#1090 fixed 10+ silent-empty-output bugs where CLI consumers of @altimateai/altimate-core read fields the engine no longer emits. The fix added a "legacy fallback" pattern to soften the shape transition, e.g.:

data.diffs ?? data.differences           // altimate-core-compare.ts
data.findings ?? data.risks              // altimate-core-migration.ts
f.suggestion ?? f.remediation             // ~8 sites across altimate-core-*.ts

The concern: these fallback branches are untested dead code. A future engine bump that renames or removes the primary field (data.diffs, data.findings, f.suggestion) silently falls through to the always-undefined legacy key and reintroduces the exact silent-empty-output bug this PR spent 10 review rounds fixing. No test catches it because the fallback branch is never exercised.

What to do

• Remove every ?? legacy_field_name fallback across packages/opencode/src/altimate/tools/altimate-core-*.ts once we've soaked 0.7.0 in prod for 1-2 releases.
• Replace with an explicit shape assertion (or a loud console.warn + telemetry event) so a future engine contract break fails loud instead of silently degrading to empty output.
• Add a "shape drift" alarm — a unit test per consumer that asserts the primary field IS present in a real engine response, so removal of it in a future engine bump breaks CI immediately.

Affected files (from the #1090 diff)

• `packages/opencode/src/altimate/tools/altimate-core-compare.ts`
• `packages/opencode/src/altimate/tools/altimate-core-migration.ts`
• `packages/opencode/src/altimate/tools/altimate-core-policy.ts`
• `packages/opencode/src/altimate/tools/altimate-core-check.ts`
• `packages/opencode/src/altimate/tools/altimate-core-query-pii.ts`
• `packages/opencode/src/altimate/tools/altimate-core-classify-pii.ts`
• `packages/opencode/src/altimate/tools/altimate-core-track-lineage.ts`
• `packages/opencode/src/altimate/tools/altimate-core-semantics.ts`

Priority

P1 — same class of bug as #1090, but latent. Address before shipping another engine bump.