docs: wire up Fumadocs Twoslash for TypeScript examples

[austenstone]

What

Wires Fumadocs Twoslash into the docs site so TypeScript code blocks tagged ts twoslash get build-time type hovers and error annotations via the TS compiler. Because Twoslash throws at build time on type errors, a twoslashed block doubles as a compile guardrail — our config examples can't drift from the real types without failing docs:build.

Twoslash only applies to TS/TSX blocks (ts twoslash), not YAML — so the 136 .actio.yml YAML samples are untouched. The real, high-value targets are the defineConfig() / actio-core TypeScript examples.

Changes

File	Change
docs/package.json / lockfile	add fumadocs-twoslash@^3.2.0 + twoslash@^0.3.9
docs/source.config.ts	add transformerTwoslash() to the Shiki rehypeCodeOptions.transformers (after the existing transformerActioKeywords())
docs/next.config.mjs	serverExternalPackages: ['typescript', 'twoslash'] (per the guide)
docs/app/global.css	@import 'fumadocs-twoslash/twoslash.css'
docs/components/mdx.tsx	register * as Twoslash from 'fumadocs-twoslash/ui' into the MDX components map (hover popups)
docs/content/docs/configuration.mdx	convert the primary defineConfig() example to ```ts twoslash

Does real actio-core resolve in Twoslash? ✅ Yes

The converted example imports defineConfig from actio-core and type-checks against the actual ActioConfig type — no paths mapping or self-contained stub needed. It resolves through the existing actio-core: file:../packages/core symlink in docs/node_modules plus the package's top-level "types": "./dist/index.d.ts" (so it resolves under any moduleResolution). Prereq: packages/core must be built (npm run build) so dist/*.d.ts exists before docs:build — already part of the documented build chain.

Verified in the built HTML for /docs/configuration: the block renders with twoslash lsp + twoslash-hover markup and the defineConfig hover surfaces the ActioConfig type.

Scope note: only one example converted (intentional)

The two custom-pass examples lower in configuration.mdx are intentionally illustrative pseudo-API — e.g. they call transformSteps(ctx, fn) whereas the real signature is transformSteps(ctx, jobId, job, fn) ("preferred form once the typed-IR API is available"). Twoslash would (correctly) reject them, so they stay as plain ts. Good follow-up once those examples match the shipped API.

Gates

• ✅ npm run docs:build — green (runs Twoslash; a type error would fail it)
• ✅ npm test -- docs-completeness syntax-reference macros — 84/84 passing
• ✅ Rendered output confirmed to contain Twoslash type-hover markup

⚠️ Merge reconciliation

This PR modifies docs/components/mdx.tsx (adds ...Twoslash to getMDXComponents). A parallel branch (austenstone-docs-clarity-overhaul) also edits this file to register <Files>/<Folder>/<File>. Both are additive entries on the same return object — trivial to combine at merge. My change is minimal/additive by design.