Fix CI action deprecation warnings

[gnezim]

Summary

• Update CI JavaScript actions to Node 24-capable releases: actions/checkout@v6, actions/setup-python@v6, and astral-sh/setup-uv@v8.2.0.
• Set Git's initial default branch config in the CI job environment so checkout no longer emits the master default-branch hint during git init.

Root Cause

The referenced run passed, but its logs had two warning sources:

• GitHub Actions warned that Node.js 20 actions are deprecated for actions/checkout@v4, actions/setup-python@v5, and astral-sh/setup-uv@v5.
• Checkout's git init emitted Git's default-branch-name hint because init.defaultBranch was not configured in the job environment.

Verification

• Confirmed replacement action manifests declare runs.using: node24 via gh api.
• Reproduced locally that GIT_CONFIG_COUNT / init.defaultBranch=main suppresses Git's default-branch hint.
• uv sync --frozen
• git diff --check
• uv run ruff check .
• uv run pytest --tb=short -q (267 passed)