feat(types,core): migrate to explicit type field discrimination

[perasperaactual]

Context

See ROADMAP.md section 'Grammar Hardening'.

Problem

The content renderer currently uses Object.entries(item)[0] to discriminate content types. This pattern:

• Relies on JS object insertion order (not guaranteed)
• Prevents TypeScript discriminated unions
• Produces poor error messages

Solution

Migrate to an explicit type field on every content item. This is a breaking change that:

• Enables proper discriminated union narrowing
• Produces clearer validation errors
• Requires updates to all YAML files, tests, and content types

Acceptance Criteria

• Update Zod schemas to use z.object({ type: z.literal('...'), ... }) pattern
• Update content renderer to use item.type switch
• Update all content type TypeScript discriminated unions
• Migrate all YAML content files to use explicit type field
• Update unit tests
• Update E2E tests
• Regenerate JSON schemas
• Update AGENTS.md tables

Timeline

This is a breaking change - coordinate with next major version bump.