feat: Add AI coding agent detection telemetry (#12159)
## Summary
- Adds a new `turborepo-ai-agents` crate that detects when `turbo` is
invoked by an AI coding agent
- Emits the detected agent name as a telemetry event (`ai_agent` key)
via `turborepo-telemetry`
- Shows the detected agent in `turbo info` output
## Detection Heuristics
Matches
[`@vercel/detect-agent`](https://github.com/vercel/vercel/tree/main/packages/detect-agent)
so Turborepo and the Vercel CLI report the same agent names.
Notable exception: OpenCode detection extends the heuristic by also
checking `OPENCODE` (which is what current OpenCode versions set), in
addition to `OPENCODE_CLIENT`.
## Testing
```bash
# See detection in turbo info
CLAUDE_CODE=1 ./target/debug/turbo info
# => AI agent: claude
# See telemetry event emission
TURBO_TELEMETRY_DEBUG=1 CLAUDE_CODE=1 ./target/debug/turbo run build --dry
# Custom agent
AI_AGENT=my-custom-agent ./target/debug/turbo info
# => AI agent: my-custom-agent
```
<sub>CLOSES TURBO-5329</sub>
