docs: explain why state_transition_history is gone (research-backed)

Adds a comment block citing a2a-sdk's own
a2a/compat/v0_3/conversions.py, which says verbatim:

  state_transition_history=None,  # No longer supported in v1.0

So a future reader who notices the missing kwarg won't try to add it
back. The capability is now universal: every v1.x Task carries a
history list and tasks/get supports historyLength via the
apply_history_length helper. No flag because nothing's optional.

Confirmed by reading the SDK source directly:
- a2a/types.py AgentCapabilities exposes only: streaming,
  push_notifications, extensions, extended_agent_card.
- a2a/compat/v0_3/conversions.py explicitly maps None when
  down-converting v1 → v0.3 (deliberate removal, not rename).
- a2a/server/request_handlers/default_request_handler_v2.py uses
  apply_history_length(task, params) — agent doesn't opt in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

workspace/main.py

@@ -188,6 +188,12 @@ async def main():  # pragma: no cover
         capabilities=AgentCapabilities(
             streaming=config.a2a.streaming,
             push_notifications=config.a2a.push_notifications,
+            # Note: state_transition_history (a 0.x capability flag) was
+            # removed in a2a-sdk 1.0. Per the SDK's own
+            # a2a/compat/v0_3/conversions.py: "No longer supported in
+            # v1.0". The capability is now universal — Task.history is
+            # always available and tasks/get accepts historyLength via
+            # apply_history_length(). Don't add this kwarg back.
         ),
         skills=[
             AgentSkill(