run --attach: detect servers that ignore the output-schema format (capability handshake) #130

From the review on #129 (deferred). aictrl run --attach --output-schema forwards format in the prompt body. A server whose PromptInput predates that field strips it and runs unconstrained. The CLI then reports structured_output: failed / missing (exit 3, no result file). That fails closed, so no invalid result is accepted, but the diagnostic is misleading.

Proposal: advertise output-schema support from the server (e.g. a capability in the config/health response, or an echo of the accepted format on the user message), and have the CLI fail fast with an explicit incompatible-server error when a schema was requested but is not acknowledged.

Note: this repo currently ships no attach HTTP server, so the target is external.