[frontend-contrib S17] AppTarget scaffolder seam + plugin resource add --app

[rickylabs]

Part of #922

Scope

ScaffolderContext gains target?: AppTarget; PluginResource entries may declare target: 'frontend'; CLI verb lands app-owned starters with provenance headers; update reports drift, never overwrites; starters never fabricate backend surface.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: ai chat-route starter migrated onto AppTarget (one generator, two callers)
• gate: drift-report-not-overwrite behavior test