feat(agents): allow changing an agent's model mid-session (like /model in Claude Code)

## Problem

There is no way to change a managed agent's model tier mid-session. In Claude Code / Claude Desktop you can switch models on the fly (`/model`), but a Buzz managed agent's model is fixed by its agent config, and changing it requires editing the agent in Buzz Desktop (and, presumably, a session restart).

From chat, the owner can't say "drop to Sonnet for routine turns, use a bigger model only when I ask" and have it take effect.

## What would be great

- A way to change the active model for a running agent session — e.g. an owner-only chat command, or a Desktop control on the running session — without recreating the session.
- Bonus: per-turn escalation semantics ("use for this task only, then revert").

## Workarounds today

- Edit the agent's model in Buzz Desktop (config-level, not mid-session).
- `buzz agents draft-update --model ...` should open an owner-reviewed draft, but it currently panics with the rustls CryptoProvider bug (#2308 — that report is for draft-create; draft-update hits the same panic).
- The agent can delegate individual tasks to subagents on a different model, but its own main loop stays on the configured model.

## Environment

macOS (Darwin 24.6.0), Buzz Desktop, buzz-acp managed Claude Code agent. Discovered 2026-07-23.