fix(complete): an attached --flag=val completes nothing #999

Both implementations complete a flag's value in the detached form and neither does in the attached one:

ex --format <cursor>     ->  json  yaml
ex --format=j            ->  (nothing)

Measured against usage-argv's complete() and against usage complete-word on this spec:

name "ex"
bin "ex"
flag "--format <F>" {
    arg "<F>" {
        choices "json" "yaml"
    }
}

Not a divergence — the two agree — so it is a gap rather than a bug in either. Worth fixing because --flag=value is a form people type and shells do ask about it; zsh and fish both send the attached word through as one token.

Left out of corpus/complete/ deliberately (#998): a vector pinning "nothing happens" would have to be deleted by whoever fixes this, and a corpus vector asserting the absence of a feature reads as a decision that it should be absent. Adding one that expects json is the natural first step of the fix.

AI-assisted — Tool: Claude Code; model: anthropic/claude-opus-5; version: unavailable.