docs: release specification 0.3.0

[dbrattli]

Summary

• Lead the quickstart with a one-line prompt that asks a coding agent to adopt ADCs.
• Explain the expected repository changes after the prompt: copy a published release locally and reference it from the agent instructions.
• Bump the specification version from 0.2.1 to 0.3.0.

Motivation

The previous quickstart began with the mechanics of copying the convention. In practice, adoption starts by asking a coding agent to adopt the repository. Showing that interaction first makes the convention easier to try while retaining the pinned-release requirements that keep adoption reproducible.

User impact

Users can begin adoption with:

Let's adopt https://github.com/dbrattli/adc for this repository.

The specification still defines the local files and instructions the agent should create.

Versioning impact

Minor release. This adds compatible adoption guidance without changing directive semantics or existing requirements.

Validation

• npx --yes markdownlint-cli2@0.23.2 --config .markdownlint.jsonc "**/*.md"
• git diff --check
• rg '^#{1,6} ' README.md AGENTS.md