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/tools/call-omo-agent/background-executor.test.ts

@@ -1,5 +1,11 @@
 /// <reference types="bun-types" />
 import { describe, test, expect, mock } from "bun:test"
+mock.module("../../shared/frontmatter", () => ({
+  parseFrontmatter: () => ({ frontmatter: {}, content: "" }),
+}))
+mock.module("js-yaml", () => ({
+  load: () => ({}),
+}))
 import type { BackgroundManager } from "../../features/background-agent"
 import type { PluginInput } from "@opencode-ai/plugin"
 import { executeBackground } from "./background-executor"
@@ -100,6 +106,35 @@ describe("executeBackground", () => {
     expect(launchArgs.fallbackChain).toEqual(fallbackChain)
   })
 
+  test("sanitizes subagent_type before passing to background manager launch", async () => {
+    //#given
+    const wrappedArgs = {
+      ...testArgs,
+      subagent_type: "\\hephaestus\\",
+    }
+    launchMock.mockResolvedValueOnce({
+      id: "test-task-id",
+      sessionId: "sub-session",
+      description: "Test task",
+      agent: "hephaestus",
+      status: "pending",
+    })
+
+    //#when
+    await executeBackground(wrappedArgs, testContext, mockManager, mockClient)
+
+    //#then
+    const latestCall = [...launchMock.mock.calls].pop()
+    if (!latestCall) {
+      throw new Error("Expected background manager launch to be called")
+    }
+    const launchArgs = latestCall[0]
+    if (!launchArgs) {
+      throw new Error("Expected launch arguments")
+    }
+    expect(launchArgs.agent).toBe("hephaestus")
+  })
+
   test("keeps launched background task alive when parent aborts before session id resolves", async () => {
     //#given - parent abort after launch should stop waiting, not fail the background task
     const abortController = new AbortController()

src/tools/call-omo-agent/background-executor.ts

@@ -8,6 +8,7 @@ import { resolveMessageContext } from "../../features/hook-message-injector"
 import { getSessionAgent } from "../../features/claude-code-session-state"
 import { getMessageDir } from "./message-dir"
 import { getSessionTools } from "../../shared/session-tools-store"
+import { sanitizeSubagentType } from "../delegate-task/subagent-discovery"
 
 export async function executeBackground(
   args: CallOmoAgentArgs,
@@ -47,7 +48,7 @@ export async function executeBackground(
     const task = await manager.launch({
       description: args.description,
       prompt: args.prompt,
-      agent: args.subagent_type,
+      agent: sanitizeSubagentType(args.subagent_type),
       parentSessionId: toolContext.sessionID,
       parentMessageId: toolContext.messageID,
       parentAgent,