Docs: document vg review findings-from-diff JSON contract (kind: correctness)

[vibgrate-team]

Context

vg review findings-from-diff --format json now emits architecture-policy rows alongside blast-radius findings as kind: correctness. Integrators need a stable field contract so CI parsers do not guess.

What to do

1. Document the JSON shape: top-level fields, finding object fields, and the meaning of kind values (at least blast-radius vs correctness / architecture-policy).
2. Note determinism: same tree + same --diff (or current change) → same ordered JSON.
3. Provide a minimal example snippet and point at actionable error cases (missing graph, empty diff).

Acceptance

• Public docs describe the JSON contract including kind: correctness
• Explicit determinism / stable-order note
• Example uses vg review findings-from-diff --format json
• DCO sign-off on the PR

Refs: #163 (broader output contracts) · release https://github.com/vibgrate/cli/releases/tag/v2026.917.1

[vibgrate-team]

Spec notes for implementers

Behavior already shipping. vg review findings-from-diff --format json emits deterministic vg.review.findings.v1 rows; architecture-policy findings appear alongside blast-radius as kind: correctness (see release notes v2026.917.1 and the README/DOCS.md review sections).

Docs gap to close.

1. Top-level document fields and ordering guarantees (same tree + same --diff / base → same JSON bytes)
2. Finding object fields, especially kind values: blast-radius vs correctness (architecture-policy)
3. Minimal example: vg review findings-from-diff --base origin/main --format json
4. Actionable failures: missing graph / empty diff — what the CLI prints and the non-zero exit

Keep this ticket to the contract write-up; do not redesign the emitter. Cross-link #163 only for broader output-contract work. DCO sign-off.