phase2-onboarding: adopt Phase 0 contract for m-test-engine

[rafael5]

Summary

Tier-2 onboarding per AI-discoverability-plan.md §3.4 and phase1-plan.md §9. Third and final tier-2 repo to ship the Phase-0 contract — completes Phase 2 onboarding after m-modern-corpus PR #2 and tree-sitter-m PR #4.

What landed

Surface	Contents
AGENTS.md	Authored from scratch — this repo had no CLAUDE.md to rename. Frontmatter declares the container's public contract (name, image base, mount point, lifecycle targets); CLAUDE.md symlinked to it. Five Phase-0 sections present.
dist/lifecycle.json	Hand-authored machine-readable description: container_name, image_base, mount_point, lifecycle (per-target make + compose forms), exec_convention (the docker exec ... bash -lc '...' shape), consumers.
dist/repo.meta.json	Phase-0 contract. Exposes lifecycle (dist/lifecycle.json) + dockerfile (docker/Dockerfile) + compose (docker/compose.yml). License AGPL-3.0.
tools/check-manifest.py	Stdlib-only validator — same pattern as tree-sitter-m.
Makefile	Three new targets: manifest (informational — lifecycle.json is hand-authored, not regenerated), check-manifest, check-docs-prose.
.github/workflows/ci.yml	Two new steps before the docker build, so a broken manifest fails fast without spinning up containers.

Verification (all green)

• make manifest — informational pointer
• make check-manifest — manifest valid + all 3 exposes.* paths present
• make check-docs-prose — no docs/ directory ✓
• Canonical Track-A validator (.github/profile/build/validate-repo-meta.py) — OK: dist/repo.meta.json

Design notes

• Hand-authored lifecycle.json (not generated). Container name, image base, mount point, and the make/compose lifecycle are static — a generator would parse the Dockerfile + compose.yml + Makefile to derive values that don't drift. AGENTS.md § Guardrails captures the "update lifecycle.json in the same commit when these change" rule.
• Manifest gate before Docker build. Cheaper to fail in 100ms on a malformed JSON than to wait for a container build. The CI ordering reflects this.

Phase 2 progress: 3 of 3 tier-2 repos onboarded with this PR.

Repo	Status
m-modern-corpus	✅ Onboarded
tree-sitter-m	✅ Onboarded
m-test-engine	✅ This PR

Phase 1 hook

Once .github ships build-catalog.py (Phase-1 Track B) and adds m-test-engine to its TIER_2 list, this repo's manifest will be picked up by the org catalog automatically — completing the parent plan's Phase 2 exit criterion ("all three tier-2 repos appear in generated tools.json with verified_on dates within 30 days").

Test plan

• All local gates green
• Canonical Track-A validator green
• CI green on this PR (manifest + docs-prose gates run before docker build, then existing smoke chain)