[bandzalkin]

Summary

This lets a host check its configured ACP backend before connecting to Band or assigning a room. await adapter.preflight(timeout_s=...) supports the generic ACP, Cursor ACP, and Copilot ACP adapters.

• Launch a throwaway process with the configured command, environment, working directory, client capabilities, and protocol options, then initialize it without creating an ACP session, room workspace, or model turn.
• Run authenticate only when auth_method is configured. This can start an interactive login or change stored credentials; leave it unset when the host must not request login.
• Return a safe PreflightResult for ordinary handshake failures. Cancellation propagates, while incomplete cleanup raises CleanupIncompleteError and stays owned for cleanup_all() to retry.
• Keep room shutdown in the requesting task, close rooms before their MCP resources, and attempt probe cleanup even when room cleanup fails or is cancelled.
• Require confirmed physical exit of retained Windows Job members as well as empty Job accounting before releasing process-tree ownership.

A successful initialize/authenticate does not establish inference entitlement, session creation, MCP connectivity, or model readiness. Band's probe diagnostics omit raw child output; third-party protocol libraries may emit their own diagnostics.

Prerequisite

Depends on the shared probe contracts in #844.

Validation

Tests cover all three adapters, configured authentication, no-room operation, real protocol peers, cancellation, late startup, cleanup retry, and diagnostic privacy.

[bandzalkin]

Closing: band-agent does not consume this; may be re-proposed per feature.