feat: typed page kinds — schema sync for `person`, `decision`, `system`

[plind-junior]

What you're trying to do

gbrain's docs/schema-author-tutorial.md walks through forking a bundled pack and adding a custom typed page kind. Vouch's PageType enum is closed (reference, decision, entity, session); contributors can't add a meeting-notes or proposal kind without forking the model. A small change with outsized leverage: let the KB declare additional page kinds in config.yaml, validated by per-kind frontmatter schemas.

Suggested shape

# .vouch/config.yaml
page_kinds:
  meeting-notes:
    frontmatter_schema: "{type: object, properties: {attendees: {type: array}}}"
    required_citations: false
  decision-record:
    extends: decision
    required_fields: [date, owner]

• vouch schema sync --apply backfills existing pages against the new kind definitions; surfaces conflicts as proposed/.
• kb.propose_page validates frontmatter against the declared kind's schema.

Acceptance

• Declaring meeting-notes lets vouch propose-page --kind meeting-notes accept that value.
• A page missing a required field fails with a per-field error.
• The bundled entity / decision / reference / session kinds keep working unchanged.

Out of scope

• Cross-kind page inheritance beyond one level.
• LLM-mediated schema inference.