AbstractMusic v0.1.8

Added

• Added abstractmusic cli as an alias for launching the interactive REPL.
• Added REPL discovery commands: /status and /engines.
• Added a download/offline toggle: CLI --download/--no-download and REPL /download on|off.

Changed

• REPL /model <id> now auto-switches the engine when the model is known in the packaged registry,
  and auto-clamps duration when a model has a declared max duration.
• REPL /models output is now an aligned table grouped by engine for readability.
• CLI default models are now sourced from the packaged model registry (via default_for_backend)
  instead of hardcoded mappings.

Docs

• Added a dedicated REPL guide (docs/repl.md) and linked it from the docs index and MkDocs nav.
• Refreshed backlog lifecycle state (moved completed/deprecated items) and expanded llms-full.txt
  to include ADR and backlog overview content for tool-friendly ingest.