Promote fleet coordination to a first-class `cao fleet` command

### Summary

Follow-up to #349 / #365. The cross-node fleet coordinator currently ships as an **examples-only** reference under `examples/fleet/`, driven by a thin, deliberately example-local `bin/fleet` helper (kept out of the installed CLI so it doesn't look like a first-class `cao-*` command — per the review discussion on #365). This issue tracks promoting that workflow to a **first-class core `cao fleet` command**.

### Proposed surface

```
cao fleet list # list nodes from the registry
cao fleet show # print the resolved API base URL
cao fleet exec # run a cao command against one node
```

This mirrors the shape @haofeif suggested on #365 and fits the established `cao session` / `cao terminal` / `cao skill` / `cao memory` / `cao workflow` pattern.

### Scope / open questions

- Register the subcommand in `cli/main.py` alongside the existing top-level commands.
- Decide the canonical node-registry location and format (the example uses `fleet.json` + the `CAO_FLEET_CONFIG` env var).
- Reconcile the word **"fleet"**: this refers to multiple *CAO nodes* (machines), whereas MCP Apps' Fleet UI (`docs/mcp-apps.md`) uses "fleet" for the set of agents on **one** `cao-server`. Naming raised by @haofeif on #365 / #367.
- The `examples/fleet` reference (node bootstrap + AI conductor + web panel) can remain as the deployment/ops example layered on top of the core command.

Raised at @haofeif's request on #365.