docs: fix the synth command, and show the async tool the scaffold writes

[ciaransweet]

Two things a reader copies that do not work. Found by checking every command and path in README.md and CLAUDE.md against the repo.

The AWS synth command fails as written

Both files documented it with one toolset in imageTags:

uv run --group infra python -m infra.cdk.app -c instance=dev \
  -c imagePrefix=ghcr.io/<owner>/<repo> -c imageTags='{"hello":"abc"}'

Every component needs a tag, index-aws and chat included, so that raises:

infra.cdk.config.ConfigError: no image tag for 'index-aws': the deploy writes
one per component to /mcp-toolsets/dev/<component>/image-tag and passes them
back as context

Both now build the map the way ci.yml already does, and say what happens if one is missing. Verified: the corrected command synthesises.

The tool example was sync

"Adding a toolset" showed def do_something(...) directly above prose telling the reader to write async def for I/O. The example is what gets copied. It is now async, which is also what the scaffold generates since #41.

The contract test refuses a sync tool that does blocking I/O since #40, so that is now listed with the other gates rather than left implicit.

What else was checked

• every ./scripts/* reference exists
• every backticked path exists, excluding URL paths, MCP method names and illustrative scaffold output
• the Quickstart commands were run against a live mcp-serve-local: the hello call and the credential-demo call with its header both return what the README shows

Nothing else was wrong.

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