fix(claudecode): strip IDE context tags from the captured turn prompt

Claude Code inside the VS Code extension prepends an <ide_opened_file> context block to the user's message. parseTurnStart stored the raw prompt, so the injected block leaked into the session/checkpoint title and rendered prompt. Strip it with textutil.StripIDEContextTags at capture, matching how the transcript and other agents already sanitize user text.

Closes #1423