Add AI SDK harness contract suite

Pin the harness dependencies and exercise host-only tools, structured output, abort, and session destruction with a scripted HarnessV1.

Assistant-model: GPT-6 Sol
Assistant-workflow: harness-stack (run f6f676b9-6506-489b-93e2-2616ea55c6a8)
Assistant-duration: 31m converged, estimated 30m
Assistant-verification: Bun contract unit passed: 3/3 fake HarnessV1 tests; broken fake variant failed 1/3 as intended
Assistant-verification: Bun ci passed: mise exec -- bun run ci (dprint, oxlint, tokens)
Assistant-verification: Bun typecheck passed: mise exec -- bun run types (all workspaces and E2E)
Assistant-verification: Bun unit passed: mise exec -- bun test (1526 pass, 2 PostgreSQL skips)
Assistant-verification: Playwright E2E failed: default-worker full runs each 227/230 with differing UI timing failures
Assistant-verification: Playwright E2E passed: E2E_SKIP_BUILD=1 mise exec -- bun run e2e --workers=1 (230/230)
Assistant-verification: agent-browser Edge manual scenario passed: @chopin reply Hello! on golden-wind; screenshot in .atomic/harness-181/evidence/slice-1
Assistant-verification: GitHub MCP spike passed: lavaman131 gh OAuth token tools({ schemas }) returned list_pull_requests,pull_request_read (App token and non-Copilot unverified)
Assistant-verification: qlty metrics passed: qlty metrics --functions apps/server/src/harness/contract.ts apps/server/src/harness/contract.test.ts
Co-authored-by: Alex Lavaee <lavaman131@github.com>