Add VS Code devcontainer

[w3lld1]

Summary

Fixes #452 by adding a VS Code Dev Container setup for SSHLibrary contributors.

Changes

• Add .devcontainer/devcontainer.json and a matching Dockerfile.
• Prepare the same SSH users/settings used by the acceptance test environment (test, test-nopasswd, testkey, banner, custom subsystem).
• Install SSHLibrary into an isolated /opt/venv during container creation and start sshd when the container starts.
• Document how to reopen the repo in the Dev Container and run acceptance tests.

Verification

• python3 -m json.tool .devcontainer/devcontainer.json
• git diff --check
• python - <<'PY' ... publish_doctree(Path('atest/README.rst').read_text()) ... PY
• Config smoke assertions for required devcontainer/Dockerfile fields.
• Ad-hoc temporary verification script under /tmp/hermes-verify-*: checked devcontainer JSON, Dockerfile semantics, README instructions, LF endings, and diff whitespace.

I could not run a full local Docker/devcontainer build in this environment because the current user cannot access /var/run/docker.sock, and passwordless sudo is unavailable. The devcontainer files were validated syntactically and with focused static/ad-hoc checks against the issue requirements above.