fix(swarm): make to_session/target_session interchangeable and normalize action synonyms

Models (even strong ones) repeatedly hit 'to_session is required' / 'target_session
is required' errors because dm/channel/message wanted to_session while assign_role/
summary/status/start/resume/etc. wanted target_session, even though both are just a
session id. Treat the two fields as interchangeable aliases in swarm tool execute().

Also map common invented action synonyms (inbox/messages -> read, send/msg -> message,
direct_message -> dm, agents -> list, plan -> plan_status, assign -> assign_task,
kill -> stop) to their canonical action instead of erroring with 'Unknown action'.

Add 'report' to the unknown-action hint list and point users at action='read' for
checking their inbox. Update schema descriptions to document the aliasing.

Addresses GH #281 / #267 swarm UX papercuts. Adds unit tests.