Persistent multi-turn Q&A / consultation command

## Problem

The current commands are all single-shot: review, adversarial-review, rescue. There's no way to have an ongoing conversation with Codex about a topic -- exploring a design, asking follow-up questions, iterating on an approach -- while maintaining context across turns.

The plugin already has thread management infrastructure (`thread/start`, `thread/resume`). Exposing this as a user-facing command would unlock conversational use cases.

## Proposal

Add `/codex:consult [topic]` command:

- Starts a new Codex thread scoped to the current repo
- Subsequent `/codex:consult [follow-up]` resumes the same thread
- `--fresh` flag starts a new thread (discards previous context)
- Session mapping persisted per-repo so threads survive across commands
- No structured verdict required -- response presented verbatim

**Use cases:**
- "How does the auth middleware work in this repo?" -> follow-up: "What would break if we replaced it with JWT?"
- Iterating on a design: ask, get feedback, refine, ask again
- Exploring unfamiliar code with a knowledgeable companion

## Implementation notes

The broker already supports `thread/start` and `thread/resume`. This would be a new command definition + prompt template + session state tracking (repo path -> thread ID mapping).