docs(core): document arg_tokenizer.rs and the per-subcommand grammar rule

Reviewer asked that this system be documented in the relevant
READMEs. Adds an Argument Tokenizer section to src/core/README.md
covering what the module provides and the design rule two confirmed
bugs this round came from: never reuse a sibling command's grammar
predicate wholesale (stash_show_wants_patch's -u/-p conflation,
requests_diff_show_raw_shape's list-drift). Cross-links from
src/cmds/git/README.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>