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/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);
+  }
+});