feat: simplify multi-select with cmd/ctrl click

[jameslcowan]

Summary

• use Cmd/Ctrl click as the primary modifier for building a multi-select instead of requiring Shift too
• let modifier-drag add a group of elements into the same pending selection before the annotation popup opens
• keep regular drag-to-annotate behavior unchanged when no modifier is held
• update the README and example copy to match the new interaction

Closes #175

Verification

• pnpm --filter agentation build
• pnpm --filter feedback-tool-example build

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