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.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,