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.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 {