fix(server,tui): restore output panel + full event history on session reopen

Reopening a session after relaunch sent only a thin summary snapshot, so the output
panel was blank and the event list showed at most 7 entries (of a 100+ event run).

- SessionSnapshot now carries lastOutput/lastResponse, resolved from the last
  InferenceCompleted artifact in CAS; tui-go onSnapshot restores the output panel
- recentEvents cap raised 7 -> 200 so a normal session's event list isn't truncated
- resolveLastOutput extracted as a helper

Note: artifact content is unaffected — verified intact and CAS-retrievable for
role_pipeline sessions (artread); content-less artifacts occur only in workflows that
record no ArtifactContentStored (e.g. healthcheck/process-result stages).

apps/tui-go/internal/protocol/protocol.go

@@ -67,6 +67,8 @@ type ServerMessage struct {
 	Reason        string  `json:"reason"`
 	Summary       string  `json:"outputSummary"`
 	Response      string  `json:"responseText"`
+	LastOutput    string  `json:"lastOutput"`   // session_snapshot: last stage output (reopen)
+	LastResponse  string  `json:"lastResponse"` // session_snapshot: last stage response (reopen)
 	Content       string  `json:"content"`
 	Diff          *string `json:"diff"`
 	Tier          string  `json:"tier"`