Simplify the Frame authoring form to the lightweight Frame body

Motivation

The Frame content model is collapsing to a single free-form markdown body (Frame Spec v0.2 requires four frontmatter fields and defines no body structure). The authoring page in web/src/pages/FrameAuthoringPage.tsx still reflects the older, heavier model: a per-slot form (terminology, rules, skills, prompts, tool_specs, goals, style, norms, architecture, business_process) plus visibility/scope/maintainer metadata fields, driven by web/src/components/form/form-model.ts and the editors in web/src/components/form/ and web/src/components/slots/.

The result is a long form that asks an author to make many decisions before they can write anything, most of which are optional and several of which no longer map to how a Frame is stored or composed. Authoring a Frame should feel like writing a document, not filling in a schema.

Proposal

• Reduce the authoring form to what a Frame actually needs: identity (name, description, version), the markdown body, and inheritance (extends/excludes).
• Drop the per-slot editors and fold their content into the body editor. Keep the .frame.md markdown source editor as the power-user path.
• Move fields that are declared metadata rather than authoring decisions (visibility, scope, maintainer) out of the primary flow - a collapsed "advanced"/metadata area, sensible defaults, or removed from the form entirely where the value is derivable.
• Delete the form components and form-model branches that are no longer reachable, so the form has one source of truth.

Acceptance criteria

• Creating and editing a Frame no longer presents the per-slot fields.
• Existing Frames (including ones published under the slot schema) open and save correctly.
• Round-tripping through the markdown source editor still works.