step 5 (complete): docs purge, fresh ARCHITECTURE + ROADMAP + CHANGELOG

Purged 29,966 lines of stale chat-switchboard documentation:
  - 41 docs/ files (ICD specs, design docs, archive, workflow docs)
  - 5 root MD files (CHANGESET, TURNOVER, ICD-DRIFT-AUDIT, etc.)

New documentation:
  - docs/ARCHITECTURE.md — kernel components, design principles, data layer
  - ROADMAP.md — v0.1.0 through v0.5.0 MVP with decision log
  - CHANGELOG.md — fresh, starting from v0.1.0 fork
  - README.md — rewritten for switchboard-core

Also in this commit:
  - config.go: stripped 7 dropped fields, DB default → switchboard_core
  - pages/loaders.go: stripped provider/model/notes/projects loaders
  - pages/pages.go: stripped persona store lookup
  - handlers/workflows.go: stripped persona tool grants
  - main.go: stripped team provider routes, avatar routes

Production code: zero references to deleted packages, stores, or models.
-29,966/+352 lines across 73 files.

server/pages/pages.go

@@ -749,12 +749,7 @@ func (e *Engine) RenderWorkflowLanding() gin.HandlerFunc {
 			if data.FirstStageMode == "" {
 				data.FirstStageMode = "custom"
 			}
-			if stages[0].PersonaID != nil {
-				if p, err := e.stores.Personas.GetByID(ctx, *stages[0].PersonaID); err == nil {
-					data.PersonaName = p.Name
-					data.PersonaIcon = p.Icon
-				}
-			}
+			// Persona lookup deferred — personas are extensions now
 		}
 
 		// Check for existing active session