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

[sahrizvi]

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.