chore(readme): fix mermaid role-flow rendering

[eschaar]

Summary

This PR contains a focused documentation/rendering fix for the role flow diagram and aligns release notes with that fix.

• Fixed the Mermaid flowchart in README.md so GitHub can render the diagram reliably.
• Updated CHANGELOG.md with a new 1.2.1 entry describing the rendering fix.
• Scope is intentionally small: only documentation/rendering, no runtime code changes.

Related Issues

No linked issue.

Validation

• Tests pass locally
• CI checks pass
• Docs updated (if needed)

Local checks run:

• Pre-commit checks passed for README.md.
• Pre-commit checks passed for CHANGELOG.md.

Release Impact

• feat: (minor)
• fix: (patch)
• BREAKING CHANGE (major)
• No release impact

[Copilot]

Pull request overview

Documentation-only PR to ensure the Mermaid role-flow diagram in README.md renders reliably on GitHub, with a corresponding changelog note.

Changes:

• Adjusted Mermaid flowchart node/label syntax in README.md for GitHub-compatible rendering.
• Added a 1.2.1 changelog entry describing the README diagram rendering fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Updates Mermaid flowchart syntax/labels to improve GitHub rendering reliability.
CHANGELOG.md	Adds a 1.2.1 entry documenting the README Mermaid rendering fix.