fix(agent-loop): compose the session prefix before pre-step; hand it to the pressure gate

ds-review-bot critical (follow-up): on the first step of a resumed or
seeded/forked instance, auto-compaction ran before runStep composed
this instance's prefix, so the gate read the PREVIOUS instance's logged
prefix from the header fold — a contributor that grew across
resume/fork (skills added, AGENTS.md grown: exactly the
environment-dependent case) could under-gate and ship an over-window
first request.

The loop now composes agent/session-prefix before the instance's first
agent/pre-step (still once per instance; runStep just reads the cache),
and agent/pre-step carries the composed prefix to its listeners.
CompactService.compactIfNeeded gains the sessionPrefix parameter;
BasicCompactService.estimatePressure gates on the handed value — the
header-fold read is gone, so the estimate is exact at every step
including a resumed/forked instance's first. Composition moving before
the boundary snapshot also means a composing listener's session append
now joins the CURRENT request (documented on the seam).

New coverage: composition precedes pre-step and the seam receives the
composed prefix; cancel and disposal landing inside the composition
window drop the step cleanly; the compact gate test hands the prefix
directly.

docs/architecture.md

@@ -70,10 +70,11 @@ forever:
     STEP loop:
       drain steering
       assemble system prompt and tool schemas
+      agent/session-prefix (first step)
       agent/pre-step
       'step/start'
       snapshot the derived messages (the reconstruction boundary)
-      agent/request (config only) -> agent/session-prefix (first request) -> log request/header -> llm/stream (frozen)
+      agent/request (config only) -> log request/header -> llm/stream (frozen)
         'assistant/chunk'
       agent/step-result
       'assistant/message'