Include conversation content in VS Code vault exports

[Railly]

Context

The Sessions save action sends only the conversation UUID. The handler defaults missing selected messages to an empty array, so the generated note contains metadata and a resume command but no transcript content.

Evidence:

• vscode/src/panels.ts:176 sends the save message.
• vscode/src/rich-extension.ts:989-1007 defaults to no selected messages.
• The note generator emits snippets only when messages are present.

Task

Export all conversation messages by default, or add an explicit message-selection flow and label the action accordingly.

Acceptance criteria

• A standard save produces a note containing conversation text.
• Empty exports require an explicit user choice.
• Metadata and resume command remain present.
• A test verifies exported human and assistant messages.