fix(mcp): fold YAML block-scalar continuations before validating the envelope

Round-6 review found a bypass in a different mechanism than rounds 1-6's
fence-detection family: frontmatterScalars's indentation guard skips any
indented continuation line outright, so old_str="half_life: durable",
new_str="half_life:\n  garbage-value" turns the field into a YAML
block-scalar shape whose real value lives on the continuation line.
present()'s own "empty value = placeholder, not an assertion" contract then
treats the field as absent, and claim.ValidateEnvelope never sees the
invalid value at all — no encoding quirk needed, one ordinary brain_edit
call smuggles an out-of-enum value straight past the check.

Fold indented continuation lines into the value when a top-level key's
inline value is empty, so oneOf() still sees what was actually written. A
genuinely empty value (no indented line follows) still folds to "",
preserving the existing placeholder-not-assertion behavior for legitimately
unset fields.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/mcp/tools_notes.go

@@ -344,7 +344,9 @@ func frontmatterScalars(doc string) map[string]string {
 	if !ok {
 		return out
 	}
-	for _, line := range strings.Split(block, "\n") {
+	lines := strings.Split(block, "\n")
+	for i := 0; i < len(lines); i++ {
+		line := lines[i]
 		if line != "" && (line[0] == ' ' || line[0] == '\t') {
 			continue
 		}
@@ -352,7 +354,31 @@ func frontmatterScalars(doc string) map[string]string {
 		if !ok {
 			continue
 		}
-		out[strings.TrimSpace(key)] = strings.TrimSpace(val)
+		val = strings.TrimSpace(val)
+		if val == "" {
+			// Round 6: a colon with nothing after it is ambiguous — it can
+			// be a genuinely empty/omitted value, or the start of a YAML
+			// block scalar whose real value lives on the indented
+			// continuation line(s) that follow (e.g. "half_life:\n
+			// durable"). Treating the latter as absent let an edit smuggle
+			// an invalid value straight past ValidateEnvelope: the field
+			// read as unset while the disk actually carried a value. Fold
+			// the continuation in as a best-effort join so oneOf() still
+			// sees it. A genuinely empty value (no indented line follows)
+			// still folds to "", preserving present()'s existing
+			// placeholder-not-assertion contract.
+			var cont []string
+			for i+1 < len(lines) {
+				next := lines[i+1]
+				if next == "" || (next[0] != ' ' && next[0] != '\t') {
+					break
+				}
+				cont = append(cont, strings.TrimSpace(next))
+				i++
+			}
+			val = strings.Join(cont, " ")
+		}
+		out[strings.TrimSpace(key)] = val
 	}
 	return out
 }