feat(mcp): capture relay tool — MCP door for non-library harnesses (#55)

Adds the `capture` MCP tool: the #55 relay for harnesses that cannot run
the use-case in-process (claude.ai Chat/Cowork/Design, Crush, Pi, LLM
Council). They reach it through the existing /mcp OAuth connector.

- Thin: forwards to the SAME CaptureService as POST /capture; holds no
  state and retains nothing beyond the I5 audit record. The containment
  properties accepted in infra security-baseline (I2 ledger) hold by
  construction.
- Per-principal: ServeHTTP re-derives the caller's principal from the
  Bearer header (the chassis middleware gates but discards it) and stashes
  it in context; the tool resolves the trust-zone origin from it. A
  caller-asserted harness/origin in the body is ignored — origin is
  server-derived, so the I1 confidential refusal still fires for us-nexus
  callers (claude.ai), and sovereign-allowlisted JWT principals pass.
- Registered only when WithCapture is wired (all three sites: tools(),
  handleCall, package doc); main wires REST + MCP from the same service,
  resolver, and credentials.

Tests: listed-only-when-wired, forwards-via-static-principal,
confidential-via-us-nexus-refused, confidential-via-sovereign-allowed,
unauthenticated-rejected, caller-cannot-forge-origin.

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

ingestion/cmd/server/main.go

@@ -413,10 +413,15 @@ func main() {
 		captureSvc := capture.NewService(
 			mcpSrv.BrainStore(), tracker, nil, classCfg, auditSink)
 		sovereign := splitList(os.Getenv("BRAIN_CAPTURE_SOVEREIGN_PRINCIPALS"))
-		captureH := capturehttp.New(captureSvc, jwtValidator, mcpToken, "local-cli",
-			capturehttp.NewOriginResolver(sovereign))
+		resolver := capturehttp.NewOriginResolver(sovereign)
+		captureH := capturehttp.New(captureSvc, jwtValidator, mcpToken, "local-cli", resolver)
 		mux.Handle("POST /capture", captureH)
-		logger.Info("capture endpoint enabled", "sovereign_principals", len(sovereign))
+		// Same use-case behind the MCP `capture` tool (#55 relay) so MCP-native
+		// harnesses (claude.ai, Crush, Pi, LLM Council) reach capture through
+		// the existing /mcp OAuth connector. mcpSrv is already wrapped above;
+		// WithCapture mutates the same instance, so the tool appears live.
+		mcpSrv.WithCapture(captureSvc, jwtValidator, mcpToken, "local-cli", resolver)
+		logger.Info("capture enabled (REST + MCP tool)", "sovereign_principals", len(sovereign))
 	} else {
 		logger.Info("capture endpoint disabled (BRAIN_GITEA_TOKEN unset)")
 	}