feat(cli): add lectio dev and zero-config scaffolding

[losolio]

What

Adds lectio dev and zero-config scaffolding to the CLI.

• lectio dev — materializes the site app and runs react-router dev: a
  local dev server with HMR on the app UI. Content is collected once at startup.
• Zero-config — when no docs.config exists, both dev and build
  scaffold a starter docs.config.ts (**/*.md, .ts not .mjs) and run with
  it, so a fresh repo shows a demo immediately.

Verified

• lectio build unchanged (regression: examples/quickstart still builds)
• Zero-config dev in a fresh repo: scaffolds docs.config.ts, collects, serves
  at :5173, route-aware (/docs, /docs/guide → 200)

Notes

• **/*.md mirrors the tree (docs/index.md → /docs); no / home unless a
  root index.md exists. The scaffolded config is a starting point to edit.
• Known edge: a root-level README.md gets a /. slug under **/*.md — left
  as-is for now.
• Content is collected once per dev run; a --watch for markdown is a natural
  follow-up.

[Copilot]

Pull request overview

This PR extends the lectio CLI with a new dev subcommand and introduces “zero-config” behavior that scaffolds a starter docs config when none is present, enabling an immediate demo experience in a fresh repository.

Changes:

• Add lectio dev, which materializes the site app and runs react-router dev for local development with HMR.
• Implement zero-config scaffolding: when no docs.config.* exists, write a starter docs.config and proceed with collect/build/dev.
• Update CLI help/docs/comments and add a changeset for a minor release.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
apps/site-builder/bin/lectio.mjs	Adds dev command, zero-config scaffolding, and routes to react-router dev vs react-router build.
.changeset/lectio-dev.md	Declares a minor release and documents the new dev + scaffolding behavior.

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