Add a venv-provisioning devcontainer per org standard

[gavinbee]

What

Add a .devcontainer/devcontainer.json that provisions a venv automatically, per the org venv standard (swimblocks/.github#29, design doc docs/design/0002-venv-standard.md).

Why

This repo documents python -m venv .venv in its README but has no devcontainer, so the venv isn't set up automatically in a Codespace/devcontainer. The standard: venv in persistent/shared environments (laptops, devcontainers), not in ephemeral CI runners.

Prerequisite

This repo currently has no requirements-dev.txt (only requirements.txt). Add one first so the devcontainer has dev deps to install — at minimum pytest; add ruff only if the repo adopts linting (note: this repo currently has no linter by design). The dev file should layer on requirements.txt (-r requirements.txt).

Baseline to include

• The Claude Code devcontainer feature (ghcr.io/anthropics/devcontainer-features/claude-code).
• A postCreateCommand that creates .venv and installs the dev requirements file.
• Point VS Code's interpreter at .venv.

Use swimblocks/.github's .devcontainer/devcontainer.json as the reference. The repo may add its own image/features/extensions on top — the standard is a floor, not an identical file.

Ongoing baseline-consistency detection is tracked in swimblocks/.github#30.