feat(types,core): interactive form content type

[perasperaactual]

Problem

There are zero interactive/input components. `contact_form_stub` renders a mailto link — it's not a form. The component library can only build brochure sites. For the "Day 365: hybrid YAML + custom React" trajectory, and for a compelling hackathon demo, the framework needs at least basic data collection capability.

Proposed approach

Add a `form` content type that renders a configurable form with typed fields:

```yaml

• form:
  label: "Contact Us"
  heading: { text: "Get in Touch", textSize: h2 }
  action: "https://formspree.io/f/xxxxx" # or mailto: or custom endpoint
  method: POST
  fields:
  - { name: "name", type: "text", label: "Your Name", required: true }
  - { name: "email", type: "email", label: "Email", required: true }
  - { name: "message", type: "textarea", label: "Message", required: false }
  - { name: "plan", type: "select", label: "Plan", options: ["Starter", "Pro", "Enterprise"] }
  submit_text: "Send Message"
  success_message: "Thanks! We'll be in touch."
  ```

Design constraints

• No backend required. Forms submit to an external endpoint (Formspree, Netlify Forms, custom URL) or use `mailto:`. Stackwright stays static.
• Schema-validated fields. Field types are a closed enum: `text`, `email`, `textarea`, `select`, `checkbox`. No arbitrary HTML.
• Client-side validation only. Required fields, email format, etc. No custom validation logic in YAML.
• Deprecates `contact_form_stub`. The new `form` type is a superset. Mark `contact_form_stub` as deprecated.

Acceptance criteria

• Zod schema for `form` content type with typed field definitions
• Core component renders accessible HTML form with proper labels and validation
• Supports external action URLs (Formspree, etc.)
• Client-side required field validation
• Success/error state handling
• Responsive layout (fields stack on mobile)
• Unit tests
• E2E test with form rendering (submission not required)
• AGENTS.md tables regenerated
• MCP screenshots include form preview