feat: update default model to potion-code-v2

[stephantul]

This PR updates the model to minishlab/potion-code-16m-v2. This model is not public yet. We should make it public before merging.

[greptile-apps]

Confidence Score: 4/5

Default model loading can fail for users without an explicit model override.

• The changed default is used when model_path is not provided.
• The loader passes that id directly to the model download path.
• The PR says the target model is not public yet.
• The new default id also differs from the model name in the PR description.

src/semble/utils.py

_{Reviews (1): Last reviewed commit: "undo exclude newer" | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/search.py	100.00% <100.00%> (ø)
src/semble/utils.py	100.00% <100.00%> (ø)
src/semble/version.py	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.