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

Models 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 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 and point users at
action='read' for checking their inbox. Update schema descriptions.

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

(The actual code changes for this; an earlier same-titled commit captured
unrelated onboarding files due to a parallel-agent staging race.)