compact: summarize is a direct one-shot llm/stream call, not a loop step

The summarization request no longer dispatches agent/request — that seam
shapes the loop's conversation requests; a hand-built one-shot's
interception surface is llm/stream, like every other direct call. The
model comes from summarizationModel falling back to the agent's own. The
turn/step parameters existed on the compact seam only to feed the
agent/request dispatch and leave compactIfNeeded/compactRegion.

Groundwork for making every conversation request a pure function of the
session log (reconstructability RFC, this branch): the seam split keeps
the loop's request path as the single thing the log must explain.

Ported from worktree-prompt-prefix-stability (PR #162) where it was
review-converged; catalog and producer/consumer graphs regenerated.

docs/cordis-catalog/services.md

@@ -79,8 +79,8 @@ Implementations MUST honor:
 - **Blocking**: no compaction begins while another is in progress for the same session. The recommended mechanism is the log-recorded lock — append `compact/start` before the slow work and `compact/end` after (even on failure) — so the lock is visible to replay and crash recovery.
 
 ```ts cordis-catalog
-abstract compactIfNeeded( agent: CompactAgentContext, turn: number, step: number, fullSystemPrompt: string, signal: AbortSignal, ): Promise<CompactionResult | null>
-abstract compactRegion( session: Session, start: number, end: number, agent: CompactAgentContext, turn: number, step: number, signal?: AbortSignal, ): Promise<CompactionResult>
+abstract compactIfNeeded( agent: CompactAgentContext, fullSystemPrompt: string, signal: AbortSignal, ): Promise<CompactionResult | null>
+abstract compactRegion( session: Session, start: number, end: number, agent: CompactAgentContext, signal?: AbortSignal, ): Promise<CompactionResult>
 ```
 
 Source: [`packages/compact/compact/src/index.ts:63`](../../packages/compact/compact/src/index.ts)