[BUG] Downstream venv path Sed transform points at the dead Debian /opt/venv path

[c-vigo]

Problem

scripts/manifest.toml:47 rewrites the workspace template's ${workspaceFolder}/.venv/bin/python3 → /opt/venv/bin/python3 (the decommissioned Debian image path). That value lands in assets/workspace/.vscode/settings.json:7, but the Nix image uses /root/assets/workspace/.venv (see assets/workspace/.devcontainer/devcontainer.json:21). Downstream projects get a broken IDE Python interpreter.

Recommendation

Update the manifest.toml transform target to the Nix venv path, regenerate, and confirm settings.json. Spot-check inside the built image that the interpreter resolves.

Context

Found during the state-of-the-repo review of #670 (epic #625).

Refs: #625