Fix Docker Build: hatchling + README context (ar-r82f.13)

[atc964]

Summary

Fix the long-failing Docker Build CI job. The root cause is a missing README.md in the runtime stage of infra/docker/Dockerfile — pip install -e . invokes hatchling's metadata validator, which reads the path declared by readme = "README.md" in pyproject.toml and raises OSError: Readme file does not exist: README.md.

Root Causes

1. README missing in runtime stage (the actual failure). Stage 2 (runtime) copies src/ and pyproject.toml from the builder but never copies README.md. The subsequent RUN pip install --no-cache-dir -e . then fails with OSError: Readme file does not exist: README.md.

2. Hatchling prepare_metadata_for_build_editable (red herring). The traceback shows AttributeError: module 'hatchling.build' has no attribute 'prepare_metadata_for_build_editable', but this is the normal pip pyproject_hooks fallback path on modern hatchling (the hook is exposed as a function, not a module attribute) and is followed by the real OSError raised during metadata validation. A defensive pin (hatchling>=1.21) is added in pyproject.toml so nothing older can be picked up.

Changes

• infra/docker/Dockerfile: add COPY --from=builder /build/README.md ./ in the runtime stage so the editable install can read it.
• pyproject.toml: pin requires = ["hatchling>=1.21"] in [build-system] (defensive).

Before / After (local docker build -f infra/docker/Dockerfile -t ad-buyer-system:test .)

Before:

#19 [runtime 8/9] RUN pip install --no-cache-dir -e .
...
OSError: Readme file does not exist: README.md
ERROR: failed to build: ... pip install --no-cache-dir -e . ... exit code: 1

After:

#20 [runtime 9/10] RUN pip install --no-cache-dir -e .
...
Successfully built ad-buyer-system
Successfully installed ad-buyer-system-0.1.0
naming to docker.io/library/ad-buyer-system:test done

Container runs cleanly: uvicorn comes up, MCP server mounts, GET /health returns 200 OK (matches the CI Docker Build job's Verify health check endpoint exists step).

Test plan

• Reproduce failure locally on origin/main (90764ab) — confirmed OSError: Readme file does not exist: README.md.
• docker build -f infra/docker/Dockerfile -t ad-buyer-system:test . succeeds locally.
• docker run -d -p 8001:8001 ad-buyer-system:test starts and /health returns 200.
• CI: Docker Build job turns green.
• CI: Test job stays green (not touched by this PR; Lint failure on main is pre-existing and out of scope).

Out of scope

• The Lint job is failing on main for an unrelated reason (ruff check on src//tests/). Not addressed here.
• infra/docker/.dockerignore is present but unused at the current build-context root; leaving it alone to avoid scope creep.

bead: ar-r82f.13