docs: update README for v0.1.2

- Code domain: promoted from 'planned' to 'second shipped domain' with full description (symbol-level merge, 11 languages, tree-sitter, DSL) - Repo structure: added code/ plugin, query_engine.py, stress test files, correct demo/ names, expanded commands list, code plugin attributes test - Dependencies: added tree-sitter, removed stale 'toml', corrected labels - Documentation section: added Code Domain link, expanded .museattributes description (six strategies, priority, comment), removed stale v0.1.1 tag - 'music query DSL' -> 'MIDI query DSL' in MIDI section - Footer: Muse v0.1.2 · Python 3.13

Co-authored-by: Gabriel Cardona <gabriel@tellurstori.com>