Keep OpenCode Python coverage fully offline

[seonghobae]

The first offline coverage bootstrap successfully builds the trusted tool image on hosted Ubuntu, but exact-head run 29551817393 exposed two execution-boundary failures after the image build:

• the low-privilege sandbox identity cannot read the root-created changed-file list;
• uv sync and uv run --with attempt PyPI resolution after --network=none is active.

Acceptance criteria:

• Keep the changed-file list root-owned and readable but not writable by UID 65532.
• Never resolve PR-selected Python dependency manifests in the networkless execution phase.
• Run coverage and pytest directly from a hash-pinned trusted image toolchain.
• Reject configured pytest commands that wrap execution in uv, Poetry, or Pipenv.
• Preserve 100% central Python coverage and publish exact failure imports when project dependencies are unavailable.