feat(swarm): default SF_AUTONOMOUS_VIA_SWARM on in headless mode

The swarm dispatch path is now automatically enabled when SF_HEADLESS=1
without requiring the operator to set SF_AUTONOMOUS_VIA_SWARM=1. This makes
headless mode use the swarm execution engine by default, which is the
intended architecture for autonomous execution.

- Explicit SF_AUTONOMOUS_VIA_SWARM=1/true still works.
- Explicit SF_AUTONOMOUS_VIA_SWARM=0/false disables it even in headless.
- When unset + SF_HEADLESS=1, swarm is used.
- When unset + SF_HEADLESS!=1, legacy path is used (unchanged).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/resources/extensions/sf/auto/run-unit.js

@@ -509,11 +509,13 @@ async function runUnitViaSwarm(ctx, _pi, s, unitType, unitId, prompt, options) {
  *     Default: false (each new unit starts with a clean session).
  */
 export async function runUnit(ctx, pi, s, unitType, unitId, prompt, options) {
-	// Feature-flagged swarm path — no-op when unset so default behavior is unchanged.
-	if (
-		process.env.SF_AUTONOMOUS_VIA_SWARM === "1" ||
-		process.env.SF_AUTONOMOUS_VIA_SWARM === "true"
-	) {
+	// Feature-flagged swarm path — default on in headless mode, opt-in elsewhere.
+	const swarmFlag = process.env.SF_AUTONOMOUS_VIA_SWARM;
+	const useSwarm =
+		swarmFlag === "1" ||
+		swarmFlag === "true" ||
+		(swarmFlag !== "0" && swarmFlag !== "false" && process.env.SF_HEADLESS === "1");
+	if (useSwarm) {
 		return runUnitViaSwarm(ctx, pi, s, unitType, unitId, prompt, options);
 	}