CLI: expose --mcp-command and --env on `buzz agents draft-create`/`draft-update` (GUI Advanced parity)

## Problem

`buzz agents draft-create` / `draft-update` can set `channel`, `display_name`,
`system_prompt`, `runtime`, `provider`, `model`, `respond_to` — but **not** the
harness's MCP command or extra env. Those are the two fields that let a managed
agent carry *external tools* (an app's own MCP server + the token it needs).

Today only Desktop's Create-agent → **Advanced** exposes them
(`desktop/src/features/agents/channelAgents.ts:237` → `runtime.mcpCommand`,
plus `env_vars` in `useGlobalAgentConfig.ts:18`). The harness already supports
an arbitrary MCP:

- `crates/buzz-acp/src/config.rs:261` — `#[arg(long, env = "BUZZ_ACP_MCP_COMMAND")] pub mcp_command: String`
- `crates/buzz-acp/src/lib.rs:4141` `build_mcp_servers()` — takes **any** command (derives name from path), not hardcoded to `buzz-cli`; injects `BUZZ_RELAY_URL` / `BUZZ_PRIVATE_KEY` into its env.
- `crates/buzz-acp/src/lib.rs:3721` — `extra_env` passthrough already exists.

So the capability is there; the CLI just can't reach it. Anyone scripting agent
provisioning (CI, an app's own `setup` command) is forced through the Desktop GUI.

## Ask

Add to `buzz agents draft-create` and `draft-update`:

```
--mcp-command # BUZZ_ACP_MCP_COMMAND for the harness
--env KEY=VALUE # repeatable; merged into the MCP subprocess env (extra_env)
```

Wire them into the same request the Desktop **Advanced** panel already sends
(`runtime.mcpCommand`, `env_vars`) — no new harness behavior, just CLI parity
with the GUI.

## Why

Lets an app provision a fully-configured managed agent from one scripted command
— e.g. a ` mcp` tool server + its `_TOKEN` — instead of walking a user
through the Advanced GUI. Same motivation as the install deep-link (block/buzz#2911):
make the agent path scriptable end-to-end.

Happy to send the PR if the shape looks right.

Start by locating the CLI entry points for `buzz agents draft-create` and `draft-update`, then compare their request construction with `desktop/src/features/agents/channelAgents.ts:237` and `useGlobalAgentConfig.ts:18`. Use `crates/buzz-acp/src/config.rs:261` and `crates/buzz-acp/src/lib.rs:3721`/`4141` to confirm the existing field names and environment behavior; done means both commands accept repeatable `--env` and `--mcp-command` and send them through the same request fields as the Desktop Advanced panel.

Written by the indexing model from the issue text.