Stale "Agentspan" product-name references in agents READMEs 🐛

The old product name **"Agentspan"** still appears in several of the agents example READMEs. It should be scrubbed from user-facing docs (the public API/error hierarchy already dropped it — see `AgentspanError` → `ConductorAgentError` in the CHANGELOG).

### Where

| File | Lines | Snippet |
|------|-------|---------|
| `examples/agents/langgraph/README.md` | 3, 63, 85, 109, 147, 162, 186 | "Add agentspan metadata", "add agentspan import", "After (**Agentspan**)" |
| `examples/agents/vercel-ai/README.md` | 68, 113 | "builds a **Agentspan** `Agent`", "// agentspan Agent + Runtime" |
| `examples/agents/adk/README.md` | 57 | "// ^^^ add agentspan import" |
| `examples/agents/openai/README.md` | 51 | "// ^^^ add agentspan import" |

### Note for whoever fixes this

Not every hit is a product-name leak. The `(graph as any)._agentspan = { ... }` metadata property (langgraph README, `docs/agents/frameworks/langgraph.md`, `langchain.md`) is a **real internal property** the serializer reads — leave that one alone. Only the prose/product-name usages ("Agentspan", "add agentspan import", "After (Agentspan)") should be reworded.

There are also a few non-README hits (`examples/agents/*.ts` comments, `AGENTS.md`, `eslint.config.mjs`) worth sweeping while you're in there — `grep -rin agentspan` finds them all.

---
Refiled on behalf of @maria-shimkovska from conductor-oss/conductor#1536 (moved here since the references live in this repo).

examples/agents 配下に一覧されている README ファイルから始め、grep -rin agentspan を実行して追加のコメントとドキュメント内の参照を見つけます。issue で示されているメタデータプロパティ _agentspan を維持しながら、文章と製品名の使用箇所だけを置き換えます。diff を確認し、serializer プロパティを変更せずに、古いユーザー向けの参照がなくなっていることを確認します。

索引モデルが issue の本文から書いたものです。