Add vine-integration-checker agent for cross-slice verification

[rdbruhn]

Context

During the vine:optimize analysis (PR #38), we identified that vine-verification handles per-slice and per-feature checks, but there's no dedicated agent for verifying cross-slice integration — that exports resolve across boundaries, data flows between modules, and API contracts hold.

Currently navigate and evolve describe this inline, but a dedicated agent would be more reliable and reusable.

Proposal

Create vine-integration-checker agent that:

• Checks imports/exports resolve across slice boundaries
• Verifies data flow between modules changed in different slices
• Flags broken references or inconsistencies
• Runs at phase group boundaries (navigate) and during evolve's cross-slice check

Inspired by

GSD's gsd-integration-checker — verifies cross-phase integration and E2E flows