fix(ci): copy tsdown config in MCP image

[AllureCurtain]

What changed

Repair the MCP OCI image build after the build-tool migration by copying the active sdown.config.ts file into the Docker build stage. Add a distribution contract assertion so the image context cannot retain the removed tsup configuration name.

Constraints and invariants

The image still installs dependencies without lifecycle scripts, builds the runtime with the configured build command, prunes development dependencies, and starts dist/mcp/index.js. Runtime behavior, API behavior, schema, and package contents are unchanged.

Out of scope

This does not change the Docker build layout, the runtime build configuration, or unrelated build-tool references.

Validation

px vitest run tests/unit/mcp-distribution.test.ts — 6 passed.

pm run typecheck — passed.

• git diff --check — passed.
• docker build -f Dockerfile.mcp -t sandbase-harness-mcp:ci . could not run locally because Docker Desktop's Linux engine was unavailable. Upstream CI runs this exact command on Linux and is the required final verification.
• Review method: self-review with CI-log and build-context tracing.