Add an account command to the CLI #1238

Background

Part of #1152. Depends on the GET /api/account endpoint.

Publishing a _validation-persist TXT record is the operator's responsibility, and much of that work happens from DNS change management or IaC pipelines rather than from a browser. The CLI needs to expose the same account URI and issuer domain names as the dashboard so the values can be scripted rather than copied by hand.

Scope

Add an account command following the existing <noun> <verb> convention:

acmebot account show [options]

• src/Acmebot.Cli/CliApplication.cs: add "account" to the command switch and a RunAccountCommandAsync
• src/Acmebot.Cli/AcmebotApiClient.cs: add GetAccountAsync() calling api/account
• src/Acmebot.Cli/Models.cs: add AccountItem, mirroring the server model
• src/Acmebot.Cli/OutputFormatter.cs: add WriteAccountAsync supporting both output formats
• src/Acmebot.Cli/HelpText.cs: add the usage line
• docs/reference/cli.md: document the command

No new options are required — the command takes only the global options, so CliOptionNames is unchanged.

Output

--format json returns the endpoint response unchanged, so it can be consumed with jq in a provisioning script.

--format table (the default) prints the fields in a readable form, with caaIdentities flattened onto one line. config show is the closest existing precedent for rendering a single object.

Acceptance Criteria

• acmebot account show prints the account URI, directory URL, and CAA identities
• --format json emits the response verbatim
• --help lists the command
• An unknown subcommand fails with the same error shape as the other commands
• Tests cover the command dispatch, the API client call, and both output formats
• The command is documented in the CLI reference