[Epic] Models/embeddings subsystem

Tracking epic for the embeddings/models pipeline: license-enforcement dev-mode carve-out, dimension-mismatch detection, local-embedding dev docs, lazy model pulling, startup backend logging, Ollama auto-detect, multimodal input support, and a derived-caching table for @embed.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

No files, tests, or entry points are named. Start by separating the listed embeddings/models workstreams and locating their existing implementation areas, then define completion criteria for each item before attempting any change.

Written by the indexing model from the issue text.