feat(vscode): add workspace prompt stash

[patroza]

Adds prompt stashing to the VS Code composer.

• persists lazily under the workspace-scoped extension storageUri instead of globalState
• scopes queues by provider, with bounded entry and attachment storage
• supports Ctrl/Cmd+S, explicit stash/restore/delete controls, images, model selection, and interaction mode
• uses atomic file replacement and preserves the live composer if persistence fails

Validation:

• vp check
• ELECTRON_SKIP_BINARY_DOWNLOAD=1 vp run -r --cache --log labeled typecheck
• pnpm --dir apps/vscode test
• pnpm --dir apps/vscode run build