Add README metadata during Composer JSON sync

[coisa]

Objective

Update UpdateComposerJsonCommand so it adds the README link to composer.json when a repository has README.md and Composer metadata does not already expose it.

Current Limitation

dev-tools:sync can normalize and enrich Composer metadata, but it does not currently add a README reference when the project already has a root README.md. As a result, packages can miss a useful Packagist/Composer metadata link even though the file exists in the repository.

Proposed Work

Teach UpdateComposerJsonCommand to detect a root-level README.md and add the Composer readme property only when it is missing.

Scope

• Detect README.md in the project root.
• Add readme to composer.json when it is absent.
• Use README.md as the default value.
• Preserve an existing readme value if one is already present.
• Keep the update idempotent across repeated dev-tools:sync runs.
• Cover the behavior with focused tests for UpdateComposerJsonCommand or the Composer JSON update service it delegates to.

Non-goals

• Generating or modifying README content.
• Overwriting an existing Composer readme value.
• Supporting multiple README filenames in the first implementation.
• Changing unrelated Composer metadata.

Acceptance Criteria

Delivery Criteria

• Running dev-tools:sync adds "readme": "README.md" to composer.json when root README.md exists and readme is missing.
• Running dev-tools:sync does not add readme when no root README.md exists.
• Existing Composer readme values are preserved unchanged.
• Re-running sync does not duplicate or reorder unrelated Composer metadata unnecessarily.
• Tests cover adding the missing README link, preserving an existing value, and skipping when the file does not exist.

Architectural / Isolation Criteria

• UpdateComposerJsonCommand remains orchestration-focused if the repository already uses a dedicated Composer JSON update helper.
• File existence checks use existing repository filesystem abstractions where practical.
• The implementation keeps deterministic JSON output compatible with existing Composer normalization.