Eliminate type:custom pages — push to Tier-4 / ADR-024

[rubenvdlinde]

8 type:custom pages out of 11 (73%). Many are inherently bespoke designer surfaces — register them in v2 5-kind registry.js with kind:'page' so the manifest entries can be properly justified with _note. Bump schema to v2.

Categorization:

• Object-backed (convert to typed index/detail): Exports (exportJob), Templates (application-template)
• Designer surfaces (keep custom + _note): Schemas, SchemaDesignerList, SchemaDesigner, PageDesigner, BuilderHost, FeaturesRoadmap

[rubenvdlinde]

Audit complete — work was already delivered via PR #110 (manifest v2 migration).

State on development (commit 416f8f2):

• $schema already v2 (app-manifest-v2.schema.json)
• Same 8/11 customs (73%), but all 8 have detailed _note justifications — the v2 escape hatch for legitimately bespoke surfaces (ADR-024)
• All 6 unique custom page components registered in src/registry.js with kind: 'page' (v2 5-kind pattern)

Why zero customs is not the right target here:
The v2 schema's type enum is [index, detail, dashboard, logs, settings, chat, files, form, wiki, map, custom] — there is no 'typed' value. The v2-correct shape for bespoke surfaces is type:'custom' + required _note + registry entry, which is already in place.

The 8 customs are inherently bespoke:

• SchemaDesigner / SchemaDesignerList / Schemas — three-pane drag-and-drop schema canvas
• PageDesigner — three-pane interactive manifest editor (REQ-OBPD-003)
• BuilderHost — meta-app-shell mounting nested CnAppRoot per virtual app
• TemplateGallery — preview-card browse + 'Use template' wizard launch
• Exports — job-queue polling list with status pills + downloads / PR links
• FeaturesRoadmap — wrapper around lib's CnFeaturesAndRoadmapView

Conversion to type:'index' would lose polling, dialog launch, gallery cards, designer canvases.

Future decomposition is blocked on a future 'designer' page type in ADR-024 (called out in several _note fields). No PR needed.