feat: per-command toggle for real-time completion (closes #776)

[palemoky]

What

Adds two zstyle options to selectively enable or disable real-time autocompletion per command, as requested in #776:

# Blacklist: autocomplete everything EXCEPT these commands.
zstyle ':autocomplete:*' disabled-commands 'ls' 'cd' 'pwd'

# Whitelist: autocomplete ONLY these commands.
zstyle ':autocomplete:*' enabled-commands 'git' 'curl' 'docker' 'kubectl'

I used zstyle rather than the plain shell variables originally sketched in the issue, to stay consistent with every other configuration option in the plugin (min-input, ignored-input, etc.).

Why

Per #776 (5 👍): users want to hide completions for commands where they aren't helpful (like ls), or keep them only where they are (like git/curl), and reduce overhead on slower systems.

How

• New helper .autocomplete:async:command-disabled, wired into .autocomplete:async:complete (the line-pre-redraw hook) before the async completion machinery starts — so suppressed commands skip the work entirely, addressing the performance motivation.
• enabled-commands (whitelist) takes precedence over disabled-commands (blacklist).
• Setting neither is a complete no-op: every command completes, exactly as today.
• Values are glob patterns (e.g. 'kube*').
• Matches both the command as typed and its base name (/usr/bin/ls → ls).
• Skips leading VAR=value assignments and precommand modifiers, so sudo ls matches ls.
• Never suppresses completion of the command name currently being typed, so command-name completion always works — including in whitelist mode.

Tests

• Existing suite: 51/51 pass.
• Manually exercised the matching logic across blacklist/whitelist, glob patterns, sudo, full paths, VAR= assignments, and command-name typing.

Closes #776