feat(brain): audit-log every tools/call; server-owned write provenance (#147)

Closes #144 (audit line per tools/call) and #145 (origin/written_by stamped on every model-reachable write, surfaced in brain_query/brain_get).

Passed pr-gatekeeper after four rounds of adversarial review, verified by execution: #147

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/mcp/audit.go

@@ -0,0 +1,48 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+	"time"
+)
+
+// logToolCall emits one structured record per tools/call (hyperguild#144,
+// OWASP MCP08): who called which tool, against which wing/hall, and how it
+// ended. Brain content is never copied into the record — only its size — so
+// the audit trail does not become a second, unscrubbed copy of the brain.
+func logToolCall(ctx context.Context, name string, args json.RawMessage, start time.Time, err error) {
+	origin, principal := provenanceFrom(ctx)
+	attrs := []any{
+		"tool", name,
+		"principal", principal,
+		"origin", origin,
+		"duration_ms", time.Since(start).Milliseconds(),
+	}
+	var a struct {
+		Wing    string `json:"wing"`
+		Hall    string `json:"hall"`
+		Content string `json:"content"`
+	}
+	if json.Unmarshal(args, &a) == nil {
+		if a.Wing != "" {
+			attrs = append(attrs, "wing", a.Wing)
+		}
+		if a.Hall != "" {
+			attrs = append(attrs, "hall", a.Hall)
+		}
+		if a.Content != "" {
+			attrs = append(attrs, "content_bytes", len(a.Content))
+		}
+	}
+	// The outcome is a class, never the error text: handler errors echo
+	// argument values (a rejected half_life, a requested path), which would
+	// make the audit line a copy of what the caller sent (PR #147 review E6).
+	// The caller already received the full error in the tool response.
+	outcome := "ok"
+	if err != nil {
+		outcome = "error"
+	}
+	attrs = append(attrs, "outcome", outcome)
+	slog.InfoContext(ctx, "mcp tool call", attrs...)
+}