## Why

The engine exposes `POST /api/v1/reindex` but there's no client command for it. Users switching embedding models must use curl directly. Adding `kb reindex` with a confirmation prompt keeps it consistent with other destructive commands like `kb remove`.

## What Changes

- Add `kb reindex` command to the Go client with confirmation prompt (skip with `--yes`/`-y`)
- Display reindex results (chunks reindexed, model used)

## Capabilities

### New Capabilities

(none)

### Modified Capabilities

- `go-client`: Add reindex command requirement

## Impact

- New file: `client/cmd/reindex.go`