feat(core/agents): add 'cloud' agent support across run_agent, /plan, /solve; docs and examples; basic tests

- core/agent_tool: recognize "cloud" model and append prompt positionally; honor per‑mode args
- core/codex(agent_run): include "cloud" in command resolution + PATH check
- core/slash_commands: default /plan,/solve models now include "cloud"; test asserts prompt includes it
- core/agent_defaults: add cloud case (no default flags); unit test ensures empty defaults
- docs: note default agent set includes "cloud"; add config example

Build passes via ./build-fast.sh. Full test suite currently fails on unrelated preexisting test drift (protocol/test-support imports, enum shapes).

docs/config.md

@@ -424,6 +424,8 @@ tool_timeout_sec = 30
 
 Sub-agents are orchestrated helper workflows you can trigger with slash commands (for example `/plan`, `/solve`, `/code`). Each entry under `[[subagents.commands]]` defines the slash command name, whether spawned agents run in read-only mode, which `agents` to launch, and extra guidance for both the orchestrator (Code) and the individual agents.
 
+By default (when no `[[agents]]` are configured) Code advertises these agent names for multi-agent runs: `claude`, `gemini`, `qwen`, `code`, and `cloud`. You can override that set by defining `[[agents]]` entries or by specifying `agents = [ … ]` on a given `[[subagents.commands]]` entry.
+
 ```toml
 [[subagents.commands]]
 name = "context"

docs/slash-commands.md

@@ -93,3 +93,7 @@ Implementation Notes
   new command, please update this document to keep the UI and docs in sync.
 - Prompt formatting for `/plan`, `/solve`, and `/code` lives in
   `codex-rs/core/src/slash_commands.rs`.
+  When no `[[agents]]` are configured, the orchestrator advertises the
+  following agent names to the LLM for multi‑agent runs: `claude`, `gemini`,
+  `qwen`, `code`, and `cloud`. You can replace or pin this set via `[[agents]]`
+  or per‑command `[[subagents.commands]].agents`.