fix(server,kernel,events): harden artifact read + restore workspace on reopen + cleanups

Follow-ups spotted while fixing the artifact-content bug:

- #1 robustness: listArtifacts resolves content via runCatching, degrading to null
  instead of failing the whole listing on one bad CAS read (resolveContent helper)
- #2 eviction visibility: WARN when a referenced artifact hash resolves to null
  (most likely evicted/compacted while still referenced by the event log)
- #3 ordering trap: document on ArtifactCreated/ArtifactContentStored that 'Created'
  fires at validation, AFTER ContentStored at inference — the latent fold hazard
- #4 reopen gap: SessionSnapshot now carries workspaceRoot (derived from
  SessionWorkspaceBound), so a reopened session restores its workspace label
- #5 orchestrator headroom: isBackEdge externalised to a top-level function so
  DefaultSessionOrchestrator drops below the TooManyFunctions threshold (was at it)

#6 (orphaned empty F-010-era artifacts) needs no code change — empty content is
already normalised to null ('(no content stored)'); emission is prevented going
forward by the shipped F-010/F-018/F-020 fixes.

apps/tui-go/internal/app/server.go

@@ -396,6 +396,7 @@ func (m *Model) onSnapshot(msg protocol.ServerMessage) {
 	}
 	sess.LastOutput = msg.LastOutput
 	sess.LastResponse = msg.LastResponse
+	sess.WorkspaceRoot = msg.WorkspaceRoot
 	for _, e := range msg.RecentEvents {
 		sess.Events = append(sess.Events, EventEntry{formatTime(e.Timestamp), e.Type, e.Detail})
 	}