New --accept-command flag to pre-approve a plugin's install command

What

Installing or updating a plugin can involve running a marketplace-declared command, which Claude Code normally asks you to confirm. A new --accept-command flag lets you pre-approve that specific command instead of using the broader -y/--yes flag.

• --accept-command <sha256> takes the sha256 hash of the exact command text (previously reported via --json's shownCommand.sha256), and only auto-accepts if it matches the command actually shown to the user for that plugin/marketplace pair; otherwise it prints a warning and leaves the command unconfirmed.
• If the marketplace's declared command has changed since the hash was generated, it refuses and re-shows the new command instead of accepting it.
• This is wired through the CLI option parser and the plugin install and plugin update command handlers alike, so both flows support scoped, non-interactive command acceptance.

Why

This lets scripts and automation approve a known, specific install command without granting blanket trust via -y/--yes, and it protects against silently accepting a different command if the marketplace's listing changes.