fix(agent): sanitize backslash/quote from agent name in launch() and background-executor

Agent names like \hephaestus\ cause 'Agent not found' errors because
sanitizeSubagentType() was only called in subagent-resolver.ts but not
in the direct manager.launch() path or background-executor.ts.

- manager.ts: strip leading/trailing backslash/quote chars from input.agent
  before validation so \hephaestus\ → hephaestus
- background-executor.ts: call sanitizeSubagentType(args.subagent_type)
  instead of passing raw value to manager.launch()
- agent-display-names.ts: reuse sanitizeSubagentType in stripAgentListSortPrefix
- Add unit tests for all three fix points

Fixes: sessions dying with 'Agent not found: \hephaestus\'

src/features/background-agent/manager.test.ts

@@ -23,6 +23,12 @@ mock.module("../../shared/connected-providers-cache", () => ({
   writeProviderModelsCache: () => {},
   updateConnectedProvidersCache: () => {},
 }))
+mock.module("../../shared/frontmatter", () => ({
+  parseFrontmatter: () => ({ frontmatter: {}, content: "" }),
+}))
+mock.module("js-yaml", () => ({
+  load: () => ({}),
+}))
 mock.restore()
 
 
@@ -2447,6 +2453,63 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
       expect(task.sessionId).toBeUndefined()
     })
 
+    test("should sanitize wrapped agent names before task creation and queueing", async () => {
+      // given
+      const input = {
+        description: "Test task",
+        prompt: "Do something",
+        agent: "\\hephaestus\\",
+        parentSessionId: "parent-session",
+        parentMessageId: "parent-message",
+      }
+
+      // when
+      const task = await manager.launch(input)
+      const queueItem = getQueuesByKey(manager).values().next().value?.[0]
+
+      // then
+      expect(task.agent).toBe("hephaestus")
+      expect(getTaskMap(manager).get(task.id)?.agent).toBe("hephaestus")
+      expect(queueItem?.input.agent).toBe("hephaestus")
+    })
+
+    test("should sanitize slash and quote wrapped agent names before task creation and queueing", async () => {
+      // given
+      const input = {
+        description: "Test task",
+        prompt: "Do something",
+        agent: "\"/hephaestus/\"",
+        parentSessionId: "parent-session",
+        parentMessageId: "parent-message",
+      }
+
+      // when
+      const task = await manager.launch(input)
+      const queueItem = getQueuesByKey(manager).values().next().value?.[0]
+
+      // then
+      expect(task.agent).toBe("hephaestus")
+      expect(getTaskMap(manager).get(task.id)?.agent).toBe("hephaestus")
+      expect(queueItem?.input.agent).toBe("hephaestus")
+    })
+
+    test("should reject wrapper-only agent names after sanitization", async () => {
+      // given
+      const input = {
+        description: "Test task",
+        prompt: "Do something",
+        agent: "\\\"/'\\\"/",
+        parentSessionId: "parent-session",
+        parentMessageId: "parent-message",
+      }
+
+      // when
+      const result = manager.launch(input)
+
+      // then
+      await expect(result).rejects.toThrow("Agent parameter is required after sanitization")
+    })
+
     test("should initialize attempt state for a newly launched task", async () => {
       // given
       const input = {

src/features/background-agent/manager.ts

@@ -383,6 +383,12 @@ export class BackgroundManager {
       throw new Error("Agent parameter is required")
     }
 
+    input = { ...input, agent: input.agent.trim().replace(/^[\\/"']+|[\\/"']+$/g, "").trim() }
+
+    if (!input.agent) {
+      throw new Error("Agent parameter is required after sanitization")
+    }
+
     const spawnReservation = await this.reserveSubagentSpawn(input.parentSessionId)
 
     try {