docs(site): document --sdui flag in installation.md

[roblevintennis]

Context

PR #431 added --sdui to ag init. The VitePress docs at v2/site/docs/installation.md need a new section covering this flag.

What to add

In v2/site/docs/installation.md, add a section (e.g. under a "Schema-Driven UI" heading) that covers:

• What the --sdui flag does and why you'd use it
• The one-liner command for each framework:

  npx agnosticui-cli init -f react --sdui
  npx agnosticui-cli init -f vue --sdui
  npx agnosticui-cli init -f lit --sdui

• What gets scaffolded (src/sdui/fixture.ts and src/sdui/SduiDemo.{tsx|vue|ts})
• How to wire SduiDemo into App
• Brief note on editing fixture.ts to customise the rendered UI
• Link to the SDUI docs/demo for further reading

Blocked by

#432 — renderer/schema packages must be published to npm before this can be documented as a working command.

File to edit

v2/site/docs/installation.md