v0.3.4 — first-turn thinking, deferred placement

v0.3.4 — first-turn thinking chip restored, placement fixed by deferral

agy attaches usage (and with it thinking_tokens) to the answer step's
DONE tail, after the text fragments already streamed. Two previous
attempts at placing the thinking annotation both failed:

• v0.3.2 annotated at DONE arrival -> the chip wedged mid-sentence
  between two text fragments;
• v0.3.3 guarded against that by skipping the annotation whenever the
  step had streamed text -> the FIRST turn of a conversation (a simple
  streamed answer) showed no thinking at all, and later tool turns then
  showed theirs, reading as "the first turn's thinking arrived late".

Now the annotation is deferred: steps that have not streamed text yet
(thinking-only turns, one-shot envelopes) annotate first as before;
streamed steps emit the chip right after the step's text completes, so
it is always present and never inside a sentence. Verified against a
live agy 1.1.15 run: fresh conversation, trivial question -> reasoning
chip present, full text intact, stop finish.