Add devcontainer contributor environment baseline

[iriswolf]

Problem

A predictable contributor environment makes OSS onboarding easier and reduces setup drift for build/test/documentation work.

Current state

The repository has CI, CodeQL, docs deployment, Dependabot, and contributor files. It does not have a .devcontainer baseline.

Target state

Add a minimal devcontainer configuration that can restore, build, test, and run documentation tooling without requiring Telegram credentials.

Acceptance criteria

• Add .devcontainer/devcontainer.json and any required supporting files.
• The container uses the supported .NET SDK version for the repository.
• Restore, build, test, and docs build commands are documented.
• No secrets or Telegram bot tokens are baked into the container.
• The setup remains optional; local development outside containers stays supported.

Non-goals

• Do not make devcontainer setup a required CI path.
• Do not add heavyweight services unless a concrete test path needs them.