docs: CLI conventions for the openab command surface

[chaodu-agent]

Docs-only. Records the CLI taxonomy rule settled in the #1449 review discussion: top-level verbs act on the bot itself (run|setup|set|get, frozen); subsystems and add-ons live under noun namespaces (openab mcp <addon> <action>); production serving is config-driven (openab run + config sections), with namespace serve actions reserved for dev/standalone use (#1451).

Review Contract

Goal

Add docs/cli-conventions.md so future CLI additions follow one documented taxonomy instead of case-by-case debate.

Non-goals

• No code or CLI changes; no renames of shipped commands (set/get stay).
• Not a style guide for flag naming or output formatting — taxonomy only.

Accepted Residual Risks

• Conventions can drift from code if unenforced; mitigated by keeping the doc to one falsifiable rule reviewers can apply mechanically.

Acceptance Criteria

• The rule classifies every existing openab subcommand without exception (reviewer check against src/main.rs Commands enum).
• The doc reflects the Relocate standalone mcp-facade entrypoint from openab-agent to the broker binary #1451 direction (config-driven serving; no new top-level serve verbs).

Follow-ups

• Update the namespace examples when Relocate standalone mcp-facade entrypoint from openab-agent to the broker binary #1451 lands (facade-only run mode) and when further openab mcp add-ons ship.