docs: require arg_tokenizer for new commands, flags and arg fixes

The module landed with no statement of when to use it, so the next command or
flag can still be written with a string scan -- which is how every bug this
migration fixed got in.

States the rule where each audience meets it: the non-negotiable list agents
load (`.claude/rules/rust-patterns.md`), the checklist for adding a filter
(`src/cmds/README.md`), and the architecture map (`ARCHITECTURE.md`), all
pointing at one place -- `src/core/README.md` -- for the four rules the
migration paid for: one grammar per subcommand, scope the lookup to the region
the tool parses, inject before the boundary, detect and act with one rule.

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