Root Markdown is not covered by any format gate and fails prettier --check

[aram356]

Problem

Root-level Markdown (CLAUDE.md / AGENTS.md, README.md, CONTRIBUTING.md,
SEQUENCE.md, …) is not covered by any formatting gate, and the agent guide
currently fails prettier --check.

The format-docs CI job runs npm run format from docs/, which is
prettier --check . — scoped to the docs/ directory. Root files are never
inspected. Verified on current main:

$ cd docs && npx prettier --check ../CLAUDE.md
[warn] ../CLAUDE.md
[warn] Code style issues found in the above file.

The drift is cosmetic — misaligned Markdown table padding in the
"Configuration Files" and "Key Files" tables, about 38 lines total. No content
is wrong.

Why file it

This is pre-existing on main, not introduced by any open PR. It is worth
recording because the gap is easy to misread: a PR that edits the agent guide
can show a green format-docs check while the file it changed was never
checked. That makes "docs format passes" misleading evidence in a test plan.

Proposed fix

Either:

1. Extend the gate — add a root-level prettier check covering *.md at the
   repo root, then run --write once to normalize the existing drift. Note
   this will touch the agent guide, so it is best landed when no large doc PR
   is in flight (see Make AGENTS.md the source of truth with CLAUDE.md as a symlink #923).
2. Or explicitly exclude root Markdown via .prettierignore and document
   that root docs are hand-formatted, so the absence of a gate is intentional
   rather than accidental.

Option 1 is preferable — the table drift is exactly the kind of thing a
formatter should own.

Context

Found while reviewing #923.