Define base-demo tooling adapter matrix

[codeforester]

Context

base-demo should become the practical test bed for Base interoperability with
adjacent developer tools, but it should not become a grab bag of active root
configuration files.

Base's current product boundary is adapter-oriented: Base owns readiness,
trust, explicit project commands, CI-safe inspection, and handoff evidence while
external tools keep their own configuration models.

Scope

• Add a base-demo tooling test-bed document that defines the lanes for adjacent
  tools.
• Classify tools by role, including direnv, asdf, chezmoi, dotbot,
  just, Taskfile, mani, gita, vcs2l, west, Docker Compose,
  Dev Containers, and Nix/devenv.
• Define which examples are active, optional, or reference-only.
• Add a contract entry and validation guard so future tool examples stay bounded.
• Cross-link the document from README and AI context.

Acceptance Criteria

• base-demo has a single documented tooling test-bed policy.
• The policy preserves the compact representative environment boundary.
• The policy states that baseline CI and Quick Start do not require the optional
  external tools unless an explicit later issue promotes one.
• tests/validate.sh checks the document and contract.

Validation

• ./tests/validate.sh
• git diff --check