Add optional TwelveLabs Pegasus video-QA tool

[mohit-twelvelabs]

Hi! I'm Mohit, I work at TwelveLabs (@mohit-twelvelabs).

This PR adds an optional TwelveLabs Pegasus video-understanding tool to the DVD agent.

What it adds

• TwelveLabsPegasusService in dvd/utils.py — a thin wrapper that uploads a local video as a TwelveLabs asset and calls Pegasus (analyze) to answer a prompt over an optional [start_time, end_time] window.
• pegasus_video_qa_tool in dvd/build_database.py — a new agent tool, wired into DVDCoreAgent next to the existing frame_inspect_tool.
• Config knobs in dvd/config.py (USE_TWELVELABS_PEGASUS, TWELVELABS_PEGASUS_MODEL_NAME, TWELVELABS_PEGASUS_MAX_TOKENS).
• README section + tests.

Why it helps DVD

DVD's deep-research QA loop reasons over very long videos by sampling still frames (frame_inspect_tool) and searching clip captions. Pegasus complements this: instead of a handful of stills, it analyzes the actual video pixels server-side, so it can answer questions about motion, ordering of events, and detail between sampled frames — exactly the cases where frame sampling falls short. It's offered as one more tool the orchestrator can choose.

Opt-in / non-breaking

• The tool is only registered when running in non-LITE mode (it needs the downloaded video pixels) and TWELVELABS_API_KEY is set.
• With the key unset, behavior is byte-for-byte identical to today. No default changed; the twelvelabs SDK is an optional install, imported lazily only when the feature is used.

How it was tested

• No-network unit tests (tests/test_twelvelabs_pegasus.py): verify the opt-in gating logic and that the tool produces a valid function schema. These always run.
• A live end-to-end test (gated on TWELVELABS_API_KEY, skipped otherwise) uploads a public sample video and asserts a non-empty Pegasus answer. I ran it locally against the real API — it passed (upload + analyze round-trip ~36s).
• Verified twelvelabs>=1.2.8 SDK signatures (assets.create, analyze, VideoContext_AssetId, response .data) match the wrapper.

Note on CLA

I understand microsoft/* repos use the Microsoft CLA bot; happy to sign it on this PR.

You can grab a free API key at https://twelvelabs.io — there's a generous free tier.

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