docs: add an "add a command" checklist to AGENTS.md

[joshrotenberg]

Part of #1049 (Phase 3, CODE-09). Docs-only; no code touched.

Problem

The enterprise command groups share one dispatch shape, but it is only discoverable by reading an existing module and inferring the convention. #1049 Phase 3 calls for writing it down.

Change

A new ## Adding a Command section in AGENTS.md, between the module-organization notes and the conventions section. It covers:

• The three-way split: clap enum, dispatch module (commands/enterprise/<name>.rs), implementation module (commands/enterprise/<name>_impl.rs).
• A checklist: declare the enum, add the EnterpriseCommands variant, write the dispatch function, write the handlers, register both modules in mod.rs, wire the match arm in main.rs, run fmt/clippy/test.
• The dispatch signature verbatim:

pub async fn handle_<name>_command(
    conn_mgr: &ConnectionManager,
    profile_name: Option<&str>,
    command: &<Name>Commands,
    output_format: OutputFormat,
    query: Option<&str>,
) -> CliResult<()>

• A handler-body sketch showing conn_mgr.create_enterprise_client(profile_name), handle_output, resolve_auto and print_formatted_output, and where those helpers come from (super::utils::*, re-exporting crate::output).

Placement conventions

Enum placement genuinely varies in the tree and both forms are current, so the checklist records both rather than declaring one wrong:

• cli/enterprise.rs as Enterprise<Name>Commands for database, cluster, node, crdb
• commands/enterprise/<name>.rs as <Name>Commands for logs, module

It suggests cli/enterprise.rs for flag-heavy groups, and notes two pre-existing variations that are not worth churning: logs keeps its dispatch function in logs_impl.rs, and some older main.rs call sites pass the command enum by value.

Checks

markdownlint AGENTS.md --disable MD013 MD033 MD041 (the flags CI uses) reports no errors in the new section. The pre-existing errors elsewhere in the file are unchanged, and that job is continue-on-error in docs.yml regardless. No Rust changed.

Relationship to #1056

Independent. #1056 converts alerts/license/license_workflow to this pattern; this PR documents the pattern. Either can land first. Together they close #1049 Phase 3.