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

@@ -214,6 +214,10 @@ describe("stripAgentListSortPrefix", () => {
   it("strips legacy zero-width sort prefixes baked into v3.14.0–v3.16.0 sessions", () => {
     expect(stripAgentListSortPrefix("\u200B\u200BHephaestus - Deep Agent")).toBe("Hephaestus - Deep Agent")
   })
+
+  it("strips leading and trailing wrapper characters after sort prefix removal", () => {
+    expect(stripAgentListSortPrefix("\\Hephaestus - Deep Agent\\")).toBe("Hephaestus - Deep Agent")
+  })
 })
 
 describe("normalizeAgentForPrompt", () => {