docs: add Vercel AI SDK and Harbor to README integrations table (#674)

## Summary
- Add the Vercel AI SDK for Python (`ai>=0.4.0`) row to the
auto-instrumented integrations table — it ships in
`py/src/braintrust/integrations/ai_sdk/` and is wired into
`auto_instrument`.
- Add the native Harbor plugin (`harbor>=0.16.0`) row — introduced in
#631, loaded by Harbor as a plugin rather than auto-instrumented.

## Test plan
- [x] Visual diff of the integrations table matches the directories
under `py/src/braintrust/integrations/` and the min-version pins in
`[tool.braintrust.matrix]`.

<!-- sfk:created-approved-by -->
Created by @AbhiPrasad

<!-- sfk:slack-thread -->
[Slack
thread](https://braintrustdata.slack.com/archives/C0AQDETAVT3/p1786383589791229?thread_ts=1786383589.791229&cid=C0AQDETAVT3)

Co-authored-by: Starfolk <noreply@starfolk.ai>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: 3 people

README.md

@@ -61,6 +61,7 @@ BRAINTRUST_API_KEY=<YOUR_API_KEY> braintrust eval tutorial_eval.py
 | [Mistral](py/src/braintrust/integrations/mistral/) | Yes | `mistralai>=1.12.4` |
 | [LiteLLM](py/src/braintrust/integrations/litellm/) | Yes | `litellm>=1.74.0` |
 | [OpenRouter](py/src/braintrust/integrations/openrouter/) | Yes | `openrouter>=0.6.0` |
+| [Vercel AI SDK for Python](py/src/braintrust/integrations/ai_sdk/) | Yes | `ai>=0.4.0` |
 | [Google GenAI](py/src/braintrust/integrations/google_genai/) | Yes | `google-genai>=1.30.0` |
 | [Bedrock Runtime](py/src/braintrust/integrations/bedrock_runtime/) | Yes | `boto3>=1.34.116` |
 | [HuggingFace Hub](py/src/braintrust/integrations/huggingface_hub/) | Yes | `huggingface-hub>=0.32.0` |
@@ -81,6 +82,7 @@ BRAINTRUST_API_KEY=<YOUR_API_KEY> braintrust eval tutorial_eval.py
 | [LiveKit Agents](py/src/braintrust/integrations/livekit_agents/) | Yes | `livekit-agents>=1.3.1` |
 | [Pipecat](py/src/braintrust/integrations/pipecat/) | Yes | `pipecat-ai>=1.3.0` |
 | [Temporal](py/src/braintrust/integrations/temporal/) | Yes | `temporalio>=1.19.0` |
+| [Harbor](py/src/braintrust/integrations/harbor/) | No (Harbor plugin) | `harbor>=0.16.0` |
 | [pytest plugin](py/src/braintrust/wrappers/pytest_plugin/README.md) | No | `pytest>=8` |
 
 ## Documentation