fix(capture): wire ai-sessions SummaryWriter into the relay (#66)

The deployed CaptureService was constructed with a nil SummaryWriter, so
a capture carrying a summary block returned the partial-failure
"no summary writer configured" (surfaced in the 2026-06-23 claude.ai
dogfood). The Gitea client already reaches mathias/* over BRAIN_GITEA_TOKEN
and now implements SummaryWriter, so inject it (type-asserted from the
tracker) — no new credential, no manifest change. Session summaries now
write to mathias/ai-sessions at summaries/<harness>/<YYYY-MM>/...

Reuses the existing token deliberately; assumes it carries contents:write
scope on ai-sessions (it already does issue writes for the tracker). If
the token is issue-scoped only, the live write 422s — a token-scope widen,
not a code fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/cmd/server/main.go

@@ -410,8 +410,12 @@ func main() {
 			os.Exit(1)
 		}
 		auditSink := buildAuditSink(ctx, brainDir, logger)
+		// The Gitea client also satisfies SummaryWriter (#66): session
+		// summaries are written to mathias/ai-sessions over the same API
+		// token. nil only if a future tracker impl lacks file writes.
+		summaryWriter, _ := tracker.(capture.SummaryWriter)
 		captureSvc := capture.NewService(
-			mcpSrv.BrainStore(), tracker, nil, classCfg, auditSink)
+			mcpSrv.BrainStore(), tracker, summaryWriter, classCfg, auditSink)
 		sovereign := splitList(os.Getenv("BRAIN_CAPTURE_SOVEREIGN_PRINCIPALS"))
 		resolver := capturehttp.NewOriginResolver(sovereign)
 		captureH := capturehttp.New(captureSvc, jwtValidator, mcpToken, "local-cli", resolver)