feat: add muse cat and muse diff --text

muse cat <file>::<symbol> [--at <ref>] Reads the blob from the object store, parses the symbol tree, matches by qualified_name then name (with ambiguity detection), and prints the exact source lines with a dim header showing address, line range, and ref. Available as both top-level `muse cat` and `muse code cat`.

muse diff --text Drops to a line-level unified diff (difflib.unified_diff) for every changed file, with coloured +/- hunks and @@ headers. Works with the same commit arguments as `muse diff`. For working-tree diffs, falls back to reading from disk when a blob is not yet in the object store.