[Bug] Agent model composer-2.5[fast=false] set in Desktop but ACP session bills/runs as fast=true

## Summary

Buzz Desktop lets me set Fizz to `composer-2.5[fast=false]`. Harness env shows `BUZZ_ACP_MODEL=composer-2.5[fast=false]`, but cursor-agent ACP actually runs `composer-2.5[fast=true]`. Cursor usage dashboard shows all agent turns as Composer 2.5 Fast.

## Environment

- Buzz Desktop (managed agents: Fizz, Bumble, Honey)
- buzz-acp harness → cursor-agent acp
- cursor-agent: 2026.07.23-e383d2b
- OS: Linux

## Steps to reproduce

1. In Buzz Desktop, set agent Fizz model to Composer 2.5 with fast=false
2. @mention Fizz in a channel
3. On the machine running buzz-acp, run:
```bash
buzz-acp models | grep -E "Current:|composer-2.5"
```
4. Check Cursor usage dashboard at cursor.com

## Expected

- ACP Current model: `composer-2.5[fast=false]`
- Usage billed as full Composer 2.5

## Actual

- Harness env: `BUZZ_ACP_MODEL=composer-2.5[fast=false]` ✓
- ACP Current: `composer-2.5[fast=true]`
- Available catalog only lists `composer-2.5 (id: composer-2.5[fast=true])` — no fast=false variant
- Cursor usage: all recent entries show Fast mode

## Screenshot


Gist: https://gist.github.com/NocturnusCoder/05290e272d734628939705d90887f83a

## Related issues

- #2692 — same silent-fallback pattern (Desktop model persists, session runs wrong model)
- #2794 / #2795 — ACP catalog id vs configId mismatch (Claude; fixed but not Composer fast=false)
- #3934 — ACP model list / discovery mismatches between Desktop and harness
- #2261 — cold-start model discovery race → silent fallback

## Notes

Possible fix: map Desktop selection to a model ID ACP actually supports, or pass `--model` at cursor-agent startup (see Cursor forum threads on ACP model switching).

Start by running `buzz-acp models | grep -E "Current:|composer-2.5"` and trace how the Desktop `BUZZ_ACP_MODEL` value reaches the cursor-agent ACP startup. Compare the requested `composer-2.5[fast=false]` with the available catalog identifier and the `--model` option mentioned in the issue. Done means ACP reports the requested model and Cursor usage is billed as full Composer 2.5 rather than Fast.

Written by the indexing model from the issue text.