feat: code domain plugin — AST-based semantic versioning (#53)

Adds muse/plugins/code/ — a first-class Muse plugin treating source code as a structured system of named symbols. Includes tree-sitter support for 10 languages, semantic content IDs, rename/move detection, symbol-level OT merge. All gates: mypy strict 0 errors, typing_audit --max-any 0, 797 tests green.