Replaces polling-only with a true server-push seam (any plugin benefits):
- core: createRouteContext accepts an emitEvent override (default still logs)
- dashboard: emitPluginCustomSseEvent forwards a plugin's ctx.emitEvent calls to
connected /api/events clients as a project-scoped 'plugin:custom' event; the
plugin route context's emitEvent is wired to it
- dashboard: PluginDashboardViewContext gains subscribePluginEvents so views
consume push via a host capability (no raw EventSource, no deep app import)
- CE view subscribes its session to push and refetches on each event; polling
stays as the fallback when push isn't wired or an event is missed
Also: skill-reachability test installs into a temp dir (no repo-dir writes).
Tests: dashboard sse +2 (plugin:custom relay + project scoping), plugin 99.
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