## Summary
Move `ConvertMessagesWithFiles` into the `g2` errgroup so prompt
conversion runs concurrently with instruction persistence, user prompt
resolution, MCP server connections, and workspace MCP tool discovery.
## Problem
In `runChat`, the setup before the first LLM `Stream()` call is
sequential across two errgroups:
```
g.Wait() // model + messages + MCP configs
ConvertMessagesWithFiles() // sequential — blocked on g2 starting
g2.Wait() // instructions + user prompt + MCP connect + workspace MCP
```
`ConvertMessagesWithFiles` can take non-trivial time on conversations
with file attachments (batch DB resolution), and it was blocking g2 from
starting.
## Fix
`ConvertMessagesWithFiles` only reads the `messages` slice (available
after `g.Wait()`) and resolves file references via the database. No g2
task reads or writes the `prompt` variable. This makes it safe to
overlap with g2:
```
g.Wait()
g2.Wait() // now includes ConvertMessagesWithFiles in parallel
```
The `InsertSystem` call for parent chats and the `promptErr` check are
deferred to after `g2.Wait()`, preserving correctness.
<details><summary>Decision log</summary>
- `ConvertMessagesWithFiles` is read-only on `messages` — no mutation,
safe for concurrent access
- `prompt` and `promptErr` are written only by the conversion goroutine,
read only after `g2.Wait()` — no data race
- Error from prompt conversion is checked immediately after `g2.Wait()`,
before any code that uses `prompt`
- `chatloop.Run` now uses `:=` instead of `=` since the prior `err`
declaration from `prompt, err :=` was removed
</details>
> Generated by Coder Agents
Kyle Carberry
8625543413