Document a minimal socket CLI input and feature-enabled workflow #147

Current status

#213 added a complete typed AST JSON example, the socket-cli build/run command, output-file behavior, and the stdout alternative to README.md. Those parts of this issue are complete. docs/ is intentionally excluded from Git; make the remaining public guidance changes in README.md or tracked command help.

Remaining scope

Difficulty: easy.

• Explain the diagnostic and recovery command when the CLI was built without socket-cli.
• Add one complete invalid input example and its expected lowering or verification failure.
• Identify the currently supported scalar subset and explain that unsupported forms fail explicitly.
• Verify examples against the actual default and socket-cli-enabled binaries, including nonzero failure status and absence of a published output on failure.

Boundaries

Do not invent a versioned transport before #54, expand the supported AST, or restore docs/ to Git. #103 owns general contribution instructions and #214 owns broken contributor-guide links.

Backlog index: #12.