[3.1] Add devcontainer / Codespaces config

[vbonk]

What needs to be done

Create .devcontainer/devcontainer.json. Language-agnostic base with commented features for Node/Python/Go/Rust. VS Code extensions. Opens README + CLAUDE.md.

Acceptance Criteria

• devcontainer.json exists
• Language features commented (user uncomments)
• init-template configures matching language

[vbonk]

Completed in Phase 3 PR #85.