usage-argv completion scripts: bash doubles a --flag= prefix, and the trailing space is wrong in both bash and zsh #1641

The completion scripts generated by usage-argv
(usage --completions <shell>, mise completion <shell>, any usage-rs CLI)
insert candidates badly, and bash and zsh are wrong in opposite ways.

1. bash: a value attached with = lands after its own prefix.

   $ usage complete-word --shell=ba<Tab>
   $ usage complete-word --shell=--shell=bash 

   The answer is the full word --shell=bash, but Readline replaces only the text after =,
   which is in the default COMP_WORDBREAKS.
   render_request (argv/src/complete.rs:1117) emits \x01prefix only for : (line 1132).
   It is the bug of Bash completion duplicates colon-prefixed task names on second Tab mise#12970, fixed by fix(bash): preserve colon-prefixed completion words #1405 and fix(completions): trim the colon prefix bash keeps for namespaced task names mise#13276 for : alone.
   The usage-cli scripts trim both (${COMP_WORDBREAKS//[^:=]/}).

2. bash: a space after every candidate, including those meant to be continued.
   mise use node<Tab> gives mise use node@ ,
   and a complete type="dir" match gives sub/ .
   The script registers with a bare complete -F (argv/src/script.rs:262),
   without the -o nospace which the usage-cli scripts, and mise up to 2026.8, had for exactly
   this reason (Does it make sense to add space after command when auto-completing? mise#4243).
   Paths behind the \x01dirs/\x01files markers are fine thanks to compopt -o filenames.

3. zsh: no space after any candidate, even a finished one.
   usage gen<Tab> gives usage generate and the cursor stays right after it,
   and the same for every subcommand, flag, choice and complete run= value.
   compadd ... -S '' (argv/src/script.rs:356) comes from fix(zsh): remove trailing space from completions and add directory slash #556,
   which fixed zsh completion adds incorrect spaces #67 (node@, /opt/homebrew/) by dropping the suffix for all candidates.

Expected in both shells: a space after a finished value (generate, --shell=bash, alpha),
none after a partial one (node@, build:, sub/), and --shell=ba<Tab> → --shell=bash.

Repro for 2 and 3 with candidates of a mise task (mkdir -p sub/inner, mise trust):

[tasks.t]
usage = '''
flag "--c <c>" { choices "alpha" "beta"; }
flag "--d <d>" { complete type="dir"; }
'''
run = 'echo "$@"'

Found while building kongusage (https://github.com/powerman/kongusage, a Kong → usage spec
exporter), which emits choices and complete type=… so that mise tasks complete better than
with the shell's file completion; with these bugs they complete worse.

Reproduced in real bash 5.3 and zsh 5.9 with usage 7.3.0 and main (220ffb2),
and with mise 2026.11.0 (usage-argv 7.3.0, as in mise 2026.11.6 and main).

AI-assisted — Tool: Claude Code; model: anthropic/claude-opus-5-5; version: 2.1.318.