Add gitbook content commands: lint, format, broken-links

[gregberge]

What

New gitbook content command group (RND-11627), bringing GitBook's own content pipeline to local markdown/git workflows:

gitbook content lint docs/                    # content GitBook would remove/restructure + invalid frontmatter
gitbook content broken-links docs/            # broken internal links & anchors (platform-exact section ids)
gitbook content format --write docs/          # apply canonical style, never alters content
gitbook content format --write --force docs/  # full normalization (exactly what GitSync commits)

The heavy lifting is @gitbook/content (companion PR: GitbookIO/gitbook-x#23807), which embeds the platform's parser, normalization and serialization — so lint reports what will actually happen on import, format output is byte-identical to GitSync exports, and anchors are validated with the same section-id algorithm the renderer uses.

Design decisions:

• lint and format have strict separate responsibilities: style is format's job (diffs shown, --write fixes), content changes are lint's job (errors with explicit messages like "This content will be removed because it is not supported by GitBook"). format never applies content-altering changes unless --force.
• The engine is an ESM-only bundle, so the CLI is now ESM: "type": "module", esbuild --format=esm with a small banner shimming require/__dirname for bundled CJS deps and the cloudflared path, bin shim uses import, postinstall.js → postinstall.cjs.
• The engine is bundled into dist/cli.js at build time but loaded lazily, so gitbook dev/auth/etc. keep their startup time.
• Exit codes are CI-friendly: errors fail, warnings only with --strict, format (check mode) fails when files need formatting.

⚠️ POC state

@gitbook/content is consumed via a file: dependency on a local gitbook-x checkout — CI will be red until the engine is published to npm. Draft until then.

Tested

• bun run build, tsc --noEmit, prettier clean.
• ESM runtime verified: --help, whoami (conf/ora/api stack), and all three content commands end-to-end (lint error → safe format preserves content → --force → lint clean).
• E2E on a real docs repo (81 files): found 2 genuine broken anchors, zero false positives.
• Not exercised: gitbook dev/check runtime (esbuild/miniflare externals import fine under ESM interop, but the dev-server path deserves a manual run before merging).

🤖 Generated with Claude Code

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