Delete @deepseek-ai/dsh-invariants, every package's ./invariant companion
and its tests, the invariant gates and Vitest setup, the scoped-event
resolver generator, and the INVARIANT-coded listener rethrow paths.
Update docs, READMEs, Agent Notes, and add an upgrade guide.
Resolve local image paths through the authenticated dsh-app file route and allow that route in rewritten Markdown images. Cover desktop rendering, previews, encoded filenames, and unsupported origins.
Fixes#5105.
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