compaction: Settings overrides + GUI divider (OPE-27 3/4)

Settings -> Models grows a Context compaction card next to Token savings:
the trigger % of the context window (10-95), the absolute token cap
(clamped 10k-2M), and the summarizer-model pin (default: the session's
own model). POST /v1/settings/compaction persists them; engines read the
knobs live per check, so changes apply to running sessions immediately.

The "context compacted" divider rides the existing notice machinery: the
persisted `compacted` notice replays on reload (itemsFromMessages) and
the live COMPACTED event appends the same info notice mid-turn. The
transcript itself stays intact - outbound-only by construction.

Covered by vitest (marker replay), a settings-card e2e (defaults +
clamped POSTs + model pin), and a mid-session divider e2e driven by the
fixtures' scripted `compacted` event.

coworker/server/app.py

@@ -1378,6 +1378,17 @@ def create_app(manager: SessionManager) -> FastAPI:
             max_mb=b.get("pdf_max_mb"),
         )
 
+    @app.post("/v1/settings/compaction")
+    def settings_set_compaction(body: dict) -> dict[str, Any]:
+        # Auto-compaction overrides (OPE-27): threshold % of the context window, the
+        # absolute token cap, and the summarizer-model pin ("" → session's own model).
+        b = body or {}
+        return manager.set_compaction_settings(
+            threshold_pct=b.get("compaction_threshold_pct"),
+            cap_tokens=b.get("compaction_cap_tokens"),
+            model=b.get("compaction_model"),
+        )
+
     @app.post("/v1/attachments/inspect-pdf")
     def attachments_inspect_pdf(body: dict) -> dict[str, Any]:
         # Attach-time page/size probe for the composer's threshold check. Local only.