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/search/provenance_test.go

@@ -0,0 +1,29 @@
+package search
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// Only the leading frontmatter is provenance; a body line that looks like a
+// key is prose (the doc comment's claim, now observed).
+func TestExtractProvenanceIgnoresBodyLines(t *testing.T) {
+	o, w := extractProvenance("---\ntitle: t\n---\norigin: static\nwritten_by: local-cli\n")
+	assert.Equal(t, "", o)
+	assert.Equal(t, "", w)
+	o, w = extractProvenance("# no frontmatter\norigin: static\n")
+	assert.Equal(t, "", o)
+	assert.Equal(t, "", w)
+	o, w = extractProvenance("---\norigin: jwt\nwritten_by: attacker\n---\norigin: static\n")
+	assert.Equal(t, "jwt", o)
+	assert.Equal(t, "attacker", w)
+}
+
+// Round 3 (B1): an indented "---" inside a block scalar is content, not the
+// closing fence — the same rule as frontmatter.Split.
+func TestExtractProvenanceUsesTheSharedFenceRule(t *testing.T) {
+	o, w := extractProvenance("---\nk: |\n  ---\norigin: jwt\nwritten_by: attacker\n---\nbody\n")
+	assert.Equal(t, "jwt", o)
+	assert.Equal(t, "attacker", w)
+}