docs: rework README into a launch ramp with cited-retrieval proof

[arhuman]

• restructure README: pain/contrast hook, "Try it in 60 seconds", real cited dialogue
• add "Does it actually retrieve?" proof section with mnemos eval numbers
• add examples/git-recipes OKF bundle as a lexical-friendly eval corpus (validated)
• move config + full command table to docs/configuration.md and docs/commands.md
• refresh demo-session.gif/tape to showcase semantic search (embed build), with a reproducibility note for the default lexical build
• fix mnemos models install usage (requires a arg)
• add "Semantic search (the embed build)" section: all-MiniLM-L6-v2 model, pure-Go inference via gomlx/onnx-gomlx (not the C++ ONNX Runtime), sugarme tokenizer, mean-pool + L2-normalize, embeddings storage, RRF (k=60) hybrid fusion, and linear-scan scaling
• link it from the README Semantic search section (kept to one line)