Documentation: API reference via @effect/docgen

[egriff38]

Scope narrowed to the API reference only. Guides and playground split out to keep this issue focused.

What lands here

API reference generated by @effect/docgen — Effect's own doc tool. Every @effect/* package uses it.

• Input: JSDoc comments on public exports (@category, @since, @example, @internal, @deprecated, description)
• Output: docs/modules/**/*.md — one file per source module, sections grouped by @category
• Enforcement: @since required, @example required, description required — configurable, on by default
• Example validation: every @example block gets typechecked against our tsconfig, so snippets can't drift from the code

Acceptance

• @effect/docgen installed and configured (docgen.json)
• JSDoc comments on every public export in src/Provider.ts, src/Executor.ts, src/Loader.ts, src/ProviderRequest.ts, src/graphiql.ts — description + @category + @since + @example per export
• bun run docgen produces valid Markdown in docs/modules/
• CI runs bun run docgen and fails if generation breaks or if an @example snippet doesn't typecheck
• Generated Markdown committed (browsable on GitHub)

Follow-ups

• chore(deps-dev): bump the effect-ecosystem group across 1 directory with 2 updates #25 — Guides (VitePress + Twoslash usage guides — the narrative content this scope originally included)
• chore(deps-dev): bump the dev-tools group across 1 directory with 4 updates #26 — Playground (Monaco + GraphiQL side-by-side, in-browser evaluation via WebContainers)

The generated API reference is v1's contract with users. The guides and playground layer on top.