Cobra's default message for a wrong positional-argument count ("accepts
2 arg(s), received 1") names neither the command nor what it expects, and
the root command sets SilenceUsage, so no usage block follows it. Running
"ocr config set provider" gave the user nothing to act on.
Add exactArgs and minimumArgs, drop-in replacements for cobra.ExactArgs
and cobra.MinimumNArgs that build the message from metadata the command
already declares: the positional signature in Use, plus Example and
ValidArgs where present. The guidance therefore cannot drift from the
command's own help output, and no command carries a hand-written error
string. This mirrors how flagErrorWithSuggestion handles the analogous
flag-error case.
Wire the seven commands that take positional arguments: config set,
config unset, rules check, session show, session comments, delegate rule
and completion. Exit codes are unchanged; only the message text differs.
The supplied count is deliberately not echoed back, since it adds nothing
the user cannot see in the line they just typed.
A tree walk over the command tree fails if a command declaring positional
placeholders still reports the raw count message, so wiring a new command
to cobra.ExactArgs directly is caught by tests.
Fixes#890
