feat: add inline --budget flag with k/m suffix support

Add a --budget shorthand for the context-token limit (--max-tokens) that
accepts a plain integer or a k/m suffix, e.g. --budget 100k == 100000.
New parseTokenBudget helper and a tokens flag type in parseGoalArguments;
invalid values are rejected with a clear error. README/CHANGELOG and unit
+ parsing tests. Item 8.1.