* refactor(backend): drop legacy orchestrator WS handlers superseded by session.launch
The orchestrator exposed six WS handlers (orchestrator.start, orchestrator.trigger,
orchestrator.prompt, orchestrator.complete, task.session.resume, task.session.prepare)
that all delegated to the unified LaunchSession or are no longer reachable from any
client. Drop the handlers, their DTOs, and the dead action constants
(orchestrator.pause, orchestrator.resume) that were never registered. Migrate the
remaining integration tests to the unified session.launch action.
* refactor(backend): drop orphan DTO request types and fix stale doc reference
Address PR review feedback: the DTO types TaskSessionStatusRequest, StopTaskRequest,
and PermissionRespondRequest had no consumers (each was shadowed by an inline
private ws*Request type in the handler). AGENTS.md still referenced the removed
task.start action; switch it to session.launch.
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Co-authored-by: Kandev Agent <agent@kandev.dev>
Carlos Florêncio