fix: remove ZWSP sort prefix from Atlas agent name

The \u200B (zero-width space) in AGENT_LIST_SORT_PREFIXES leaked into
the x-opencode-agent-name HTTP header, causing invalid header errors.

Agent ordering is handled by agent-priority-order.ts via numeric order
fields and insertion order — the ZWSP prefix was redundant.

src/shared/agent-display-names.test.ts

@@ -185,8 +185,8 @@ describe("getAgentListDisplayName", () => {
     expect(getAgentListDisplayName("sisyphus")).toBe("Sisyphus (Ultraworker)")
   })
 
-  it("applies invisible atlas sort prefix for list display", () => {
-    expect(getAgentListDisplayName("atlas")).toBe("\u200BAtlas (Plan Executor)")
+  it("returns atlas display name without sort prefix", () => {
+    expect(getAgentListDisplayName("atlas")).toBe("Atlas (Plan Executor)")
   })
 })