Closes/relates to Runfusion/Fusion#1956.
## Summary
Surfaces silent runtime-resolution fallback in the dashboard, and
threads the real `FallbackReason` ("not_found" vs "factory_error")
through instead of hardcoding `"not_found"` for every fallback.
## Changes
- `packages/engine/src/runtime-resolution.ts`: `resolvePluginRuntime()`
now returns a tagged miss result (`{ ok: false, reason }`)
distinguishing "not found" from "factory/instantiation error" instead of
collapsing both to `null`. `resolveRuntime()` threads the real reason
through to `logRuntimeFallback(...)` and returns it via
`ResolvedRuntime.fallbackReason`.
- `packages/engine/src/agent-session-helpers.ts`:
`createResolvedAgentSession()` includes `fallbackReason` in the
`session:runtime-resolved` audit event metadata when present.
- `packages/dashboard/src/routes/register-task-workflow-routes.ts`: new
`GET /api/tasks/:id/runtime-fallback` endpoint, returning the most
recent `session:runtime-resolved` event normalized for UI consumption
(`wasConfigured`, `runtimeHint`, `reason`, `showFallbackBadge`).
- `packages/dashboard/app/hooks/useRuntimeFallbackStatus.ts` (new):
polls the endpoint, dedupes toast firing per audit-event-id.
- `packages/dashboard/app/components/RuntimeFallbackBadge.tsx` (new):
renders the badge + fires the toast; wired into `TaskCard.tsx`,
`ActiveAgentsPanel.tsx`, and `AgentsView.tsx` (board and list variants).
## Test plan
- `pnpm --filter @fusion/engine exec vitest run
src/__tests__/runtime-resolution.test.ts` — 24/24 pass (21 pre-existing
+ 3 new, none weakened)
- `pnpm --filter @fusion/dashboard exec vitest run
src/routes/__tests__/register-task-workflow-routes.runtime-fallback.test.ts`
— 5/5 pass
(empty/configured-ok/fallback-with-hint/fallback-blank-hint/stale-superseded
states)
- `pnpm --filter @fusion/dashboard exec vitest run
app/components/__tests__/RuntimeFallbackBadge.test.tsx` — 8/8 pass (all
data states + mobile breakpoint + toast-fires-once)
- `pnpm --filter @fusion/dashboard run typecheck` and `pnpm --filter
@fusion/engine run typecheck` — both clean
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added runtime-fallback warning badges across task and agent views
(including board and “working on” sections) with automatic toast
notifications.
* Introduced a new backend API to surface the latest runtime-fallback
state for a task.
* Added runtime-fallback status polling and UI messaging to reflect the
most recent state.
* **Bug Fixes**
* Prevented repeated toasts by deduplicating notifications across
polling updates.
* Improved fallback reporting so the UI reflects the latest
runtime-resolved audit event.
* Enhanced diagnostics by distinguishing fallback reasons (e.g., missing
runtime vs factory failure) for clearer user guidance.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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