fix: make /dispatch-list-incidents arguments and project scoping reachable

[Jamyn]

The handler split payload["command"] — the command name Slack sends, never the arguments — so the two-element branch was unreachable and a project argument was silently ignored. Arguments now come from payload["text"], unsplit, so a project name containing spaces works; the not-found error reports the request's own organization instead of inventing one from args[0].

Project scoping was equally unreachable: the command carried no subject, so it always read the default organization. A new optional_command_context_middleware resolves the conversation when there is one — adopting both its subject and its session — and falls through outside a channel, where the required variant would have raised.

Closes #133.