Add setting to make active-file/selection context opt-in (manual) instead of automatic

Currently the VSCode extension automatically sends the currently open editor file, and any selected lines within it, as context to Claude on every message — with no setting to disable this.

Requested: a setting (or UI toggle) to make this opt-in — i.e. don't auto-include the open file/selection by default, but still expose it as a manual option (e.g. a button/checkbox to attach it when desired, similar to how @-mentions work for other files).

Confirmed there is currently no claudeCode.* setting in package.json's contributes.configuration that controls this behavior.

Environment: VSCode 1.136.0, Claude Code extension 2.1.260, Linux