feat: implement postMessage for BacklogAdapter

[amotarao]

Summary

• BacklogAdapter.postMessage を実装
• decodeThreadId で issueKey を取得し、BacklogClient.postComment で Backlog API にコメントを投稿
• AdapterPostableMessage を renderPostable で Backlog markup に変換してから投稿

Test plan

• postMessage が正しい issue にコメントを POST し、RawMessage<BacklogComment> を返すこと
• markdown メッセージが Backlog markup に変換されてから投稿されること

Closes PC-26

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9498377a-5586-41a0-b8e5-d073deac2310

📥 Commits

Reviewing files that changed from the base of the PR and between 887fdeb and 4ff05ef.

📒 Files selected for processing (3)

• .changeset/pc-26-postmessage.md
• packages/chat-adapter-backlog/src/adapter.test.ts
• packages/chat-adapter-backlog/src/adapter.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch amon/pc-26-implement-postmessage-for-backlogadapter

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