[frontend-contrib S14] netscript plugin dev (frontend watch loop)

[rickylabs]

Part of #922

Scope

Watches frontend/, maintains the export map, regenerates the host registry atomically, signals vite reload when island inputs change; documents local-source vs JSR loops honestly.

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

Acceptance

• gate: edit route -> hot reload; add island -> regen + signaled reload (local-source)
• gate: export map never hand-maintained in the documented flow