Feat v0.3.3 public entry background jobs (#17)

Co-authored-by: Jeffrey Smith <jasafpro@gmail.com>
Co-committed-by: Jeffrey Smith <jasafpro@gmail.com>

CHANGELOG.md

@@ -2,6 +2,27 @@
 
 All notable changes to Switchboard Core are documented here.
 
+## v0.3.3 — Public Entry + Background Jobs
+
+### Added
+
+- **Public workflow entry**: Unauthenticated routes for anonymous workflow
+  participation. `POST /api/v1/public/workflows/:id/start` creates an instance
+  with `started_by = "public:<uuid>"` and returns an entry token.
+  `GET .../resume/:token` and `POST .../advance/:token` allow continuation.
+  Only stages with `audience = "public"` can be advanced anonymously.
+- **SLA scanner**: Background goroutine (5-minute interval) checks active
+  instances against per-stage `sla_seconds`. Fires `workflow.sla_breach` event
+  on first breach, marks `sla_breached: true` in instance metadata (idempotent).
+- **Staleness sweep**: New `staleness_timeout_hours` column on `workflows`.
+  Scanner marks instances as `stale` when `updated_at` exceeds the threshold,
+  cancels open assignments, fires `workflow.stale` event.
+- **New store methods**: `ListActiveInstances()`, `MarkInstanceStale()`.
+- **New events**: `workflow.sla_breach`, `workflow.stale`.
+- **3 new store tests**: MarkStale, ListActive, StalenessTimeoutHours round-trip.
+
+---
+
 ## v0.3.0 — Workflow Schema Redesign
 
 ### Changed