fix(text): truncate on rune boundaries, not byte offsets (#93)

Four call sites did `s[:limit] + "…"`. Both halves are byte operations:
len() counts bytes and a slice cuts bytes. On Swedish text — å ä ö are
two bytes, an em dash three — the cut lands mid-character and leaves an
orphaned lead byte, producing invalid UTF-8.

It failed loudly in exactly one place: wiki/index.md is written raw to
disk, where every strict decoder then rejects the whole file. The other
three (brain_context excerpts x2, brain_query excerpts) feed MCP
responses, where encoding/json silently substitutes U+FFFD — so those had
been quietly eating the last character of truncated excerpts, with no
error anywhere and nothing to notice.

textutil.Truncate counts runes and appends the marker only when content
was actually dropped; an ellipsis on complete text is a lie about the
content. The test is a property, not an example: it walks the limit
across every offset of a multi-byte string and asserts utf8.Valid at each
one, so a future off-by-one cannot pass by picking a lucky boundary.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m

ingestion/internal/textutil/truncate.go

@@ -0,0 +1,45 @@
+// Package textutil holds small text helpers shared across the ingestion
+// service.
+package textutil
+
+import "unicode/utf8"
+
+// Ellipsis marks a value that was cut short.
+const Ellipsis = "…"
+
+// Truncate shortens s to at most limit CHARACTERS, appending an ellipsis when
+// anything was dropped.
+//
+// It exists because four call sites did `s[:limit] + "…"` (#93). Both halves
+// of that are byte operations: len() counts bytes and a slice cuts bytes, so
+// on Swedish text — where å ä ö are two bytes and an em dash is three — the
+// cut lands mid-character and leaves an orphaned lead byte. The result is
+// invalid UTF-8.
+//
+// That failed loudly in exactly one place, wiki/index.md, which is written
+// raw to disk and then rejected by every strict decoder. The other three fed
+// MCP responses, where encoding/json silently substitutes U+FFFD — so those
+// had been quietly eating the last character of truncated excerpts with no
+// error anywhere.
+//
+// The marker is appended only when content was actually dropped: an ellipsis
+// on complete text is a lie about the content.
+func Truncate(s string, limit int) string {
+	if limit <= 0 {
+		return ""
+	}
+	if utf8.RuneCountInString(s) <= limit {
+		return s
+	}
+
+	// Walk runes rather than converting to []rune: the input can be a whole
+	// note body, and this runs on the read path.
+	count := 0
+	for i := range s {
+		if count == limit {
+			return s[:i] + Ellipsis
+		}
+		count++
+	}
+	return s + Ellipsis
+}