Apply Markdown formatting options through Prettier config

[Eugene (OEvgeny)]

Changelog Entry

Not required, development only.

Description

Markdown uses 3-space indentation, but that rule only lived in lint-staged.config.js (prettier --write <file> --tab-width 3 --single-quote true). Plain Prettier and editors read .prettierrc.yml, which says tabWidth: 2. So npx prettier --check CHANGELOG.md fails on main, while the pre-commit hook formats the same file without changes. Contributors and agents repeatedly spent time proving that this failure was "pre-existing" before they could commit.

This PR moves the Markdown option into the Prettier config, so Prettier, editors, and the pre-commit hook agree. No Markdown file is reformatted.

Design

• .prettierrc.yml gets an overrides entry for *.md with tabWidth: 3. singleQuote is already true, so the second CLI flag was redundant.
• The 11 nested .prettierrc.yml files (samples/ and 10 packages) are identical copies of the root config. Prettier uses the nearest config file, so they get the same override. Without it, Markdown files in those folders would still format with 2 spaces under plain Prettier.
• lint-staged now runs plain prettier --write for Markdown, like it does for code.

Formatting output is unchanged. For each of the 137 tracked Markdown files, the old hook command on main and plain prettier on this branch produce byte-identical output. The same 49 files pass prettier --check, including CHANGELOG.md.

Many Markdown files (85, e.g. README.md, AGENTS.md, most of docs/ and samples/) already fail both the old hook form and plain Prettier, because they predate it or were committed without the hook. This PR leaves them as they are; the hook will format them when they are next edited, as it does today.

Specific Changes

• Added a *.md override with tabWidth: 3 to .prettierrc.yml, samples/.prettierrc.yml, and packages/*/.prettierrc.yml

• lint-staged.config.js: Markdown uses prettierCode, and the prettierMarkdown function and its CLI flags are removed

• I have added tests and executed them locally

• I have updated CHANGELOG.md

• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

🟢 Approval recommended

The configuration changes consistently replace the hook-specific options, with no blocking issues identified in static review.

No open issues 🎉

What changed in this PR

Aligns Markdown formatting across Prettier, editors, and pre-commit hooks without reformatting Markdown files.

Changes:

• Adds three-space Markdown indentation to all 12 Prettier configurations.
• Reuses the standard lint-staged formatter, removing redundant command-line options.

File	Description
samples/​.prettierrc.yml	Adds Markdown indentation override.
packages/​playground/​.prettierrc.yml	Adds Markdown indentation override.
packages/​isomorphic-react/​.prettierrc.yml	Adds Markdown indentation override.
packages/​isomorphic-react-dom/​.prettierrc.yml	Adds Markdown indentation override.
packages/​fluent-theme/​.prettierrc.yml	Adds Markdown indentation override.
packages/​embed/​.prettierrc.yml	Adds Markdown indentation override.
packages/​directlinespeech/​.prettierrc.yml	Adds Markdown indentation override.
packages/​core/​.prettierrc.yml	Adds Markdown indentation override.
packages/​component/​.prettierrc.yml	Adds Markdown indentation override.
packages/​bundle/​.prettierrc.yml	Adds Markdown indentation override.
packages/​api/​.prettierrc.yml	Adds Markdown indentation override.
lint-staged.config.js	Uses the shared formatter for Markdown.
.prettierrc.yml	Sets repository-wide Markdown indentation.

_{Review effort: Balanced}