fix(frontmatter): consolidate 3 independent fence-parsers into one (#139)

Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/mcp/tools_notes.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/claim"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/classification"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/frontmatter"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/notes"
 )
 
@@ -331,27 +332,20 @@ func checkClaimEnvelope(brainDir, path, oldStr, newStr string) error {
 // frontmatterBlock returns the raw text between doc's opening and closing
 // `---` fences, or ok=false if doc has no (properly closed) fence.
 func frontmatterBlock(doc string) (block string, ok bool) {
-	// Round 4 found a note with CRLF fence lines ("---\r\n") silently
-	// defeats HasPrefix(doc, "---\n") on the precondition read — not an
-	// adversarial old_str/new_str, just an encoding difference — which
-	// makes checkClaimEnvelope no-op entirely. Round 5 found the same
-	// mechanism one layer earlier: a leading UTF-8 BOM (a live, plausible
-	// byte sequence — Notepad's default "UTF-8" save and PowerShell's
-	// pre-v6 Out-File/Set-Content both emit one) defeats the same prefix
-	// check before CRLF normalization even applies. Strip both, here, so
-	// every caller (hadFence/hasFence detection and frontmatterScalars)
-	// agrees regardless of encoding.
-	doc = strings.TrimPrefix(doc, "\xef\xbb\xbf")
-	doc = strings.ReplaceAll(doc, "\r\n", "\n")
-	if !strings.HasPrefix(doc, "---\n") {
+	// hyperguild#138: fence detection itself is frontmatter.Split, shared
+	// with internal/api and internal/frontmatter — the CRLF/BOM guards that
+	// used to live here (added piecemeal across rounds 4 and 5 of #123's
+	// review) are now the shared parser's job, so every caller inherits a
+	// fix instead of needing bug-for-bug parity edits.
+	b, _, ok := frontmatter.Split([]byte(doc))
+	if !ok {
 		return "", false
 	}
-	rest := doc[len("---\n"):]
-	end := strings.Index(rest, "\n---\n")
-	if end < 0 {
-		return "", false
-	}
-	return rest[:end], true
+	// Split's block keeps the "\n" immediately before the closing fence
+	// (by design — see its own doc comment); this function never included
+	// it. Split guarantees ok=true only when "\n---\n" matched, so this is
+	// always exactly one byte, never ambiguous.
+	return strings.TrimSuffix(string(b), "\n"), true
 }
 
 // frontmatterScalars extracts top-level "key: value" scalar pairs from