swarm ui: a soft context limit field in an agent's inference settings

A whole percent from 5 to 100 of the model's context window, sent as
soft_context_percent next to max_subagents. The inference row shows it, the
edit dialog and the create form take it, and an archetype can carry it.

frontend/packages/swarm-ui/test/create-agent-archetype.test.mjs

@@ -15,7 +15,13 @@ const empty = {
   permissions: defaults,
   cpu: "",
   memory: "",
-  inference: { model: "", effort: "", subagents: "", maxSubagents: "" },
+  inference: {
+    model: "",
+    effort: "",
+    subagents: "",
+    maxSubagents: "",
+    softContext: "",
+  },
 };
 
 test("no archetype is the default permissions and empty settings", () => {
@@ -34,6 +40,7 @@ test("an archetype's settings fill the form", () => {
       effort: "high",
       subagent_models: ["haiku", "sonnet"],
       max_subagents: 0,
+      soft_context_percent: 20,
     },
   };
   assert.deepEqual(prefill(reviewer, defaults), {
@@ -45,6 +52,7 @@ test("an archetype's settings fill the form", () => {
       effort: "high",
       subagents: "haiku, sonnet",
       maxSubagents: "0",
+      softContext: "20",
     },
   });
 });

frontend/packages/swarm-ui/test/soft-context.test.mjs

@@ -0,0 +1,33 @@
+// The soft context limit field: what counts as empty, valid and refused. Run
+// by `npm test`, which imports the `.ts` module as it is.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  MAX_SOFT_CONTEXT_PERCENT,
+  MIN_SOFT_CONTEXT_PERCENT,
+  softContextValue,
+} from "../src/agent/detail/soft-context.ts";
+
+test("an empty box leaves the agent on its own limit", () => {
+  assert.equal(softContextValue(""), undefined);
+  assert.equal(softContextValue("   "), undefined);
+});
+
+test("a whole percent from 5 to 100 is taken", () => {
+  assert.equal(softContextValue("20"), 20);
+  assert.equal(softContextValue(" 75 "), 75);
+  assert.equal(
+    softContextValue(String(MIN_SOFT_CONTEXT_PERCENT)),
+    MIN_SOFT_CONTEXT_PERCENT,
+  );
+  assert.equal(
+    softContextValue(String(MAX_SOFT_CONTEXT_PERCENT)),
+    MAX_SOFT_CONTEXT_PERCENT,
+  );
+});
+
+test("anything else is refused", () => {
+  for (const text of ["0", "4", "101", "-20", "20.5", "abc", "1e1", "20%"]) {
+    assert.equal(softContextValue(text), null, text);
+  }
+});