fix(vectorstore): hard-split oversized heading-less chunks

A doc with no headings and no blank-line paragraphs (JSON-lines, e.g.
wiki/telos/decisions/human-intent-column.md) survived both chunk passes whole
and was sent to nomic-embed over its context window → 'input length exceeds
the context length' (400, the steady embed errors=1). Add a final hard-split
pass (line then UTF-8 rune boundaries) so no chunk exceeds maxBytes. TDD.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/vectorstore/chunk_test.go

@@ -30,6 +30,23 @@ func TestChunkMarkdown_SplitsAtHeadings(t *testing.T) {
 	}
 }
 
+func TestChunkMarkdown_HardSplitsHeadinglessOversizedBlock(t *testing.T) {
+	// A document with no headings and no blank-line paragraph breaks (e.g.
+	// JSON-lines like wiki/telos/decisions/human-intent-column.md). The old
+	// chunker emitted it as one over-budget chunk → nomic-embed returned
+	// "input length exceeds the context length" (400). Every chunk must now
+	// fit the budget, with no content lost.
+	maxBytes := 200
+	src := strings.Repeat("x", 1000) // one 1000-byte blob, no headings, no \n\n
+	out := vectorstore.ChunkMarkdown(src, maxBytes)
+
+	require.Greater(t, len(out), 1, "oversized blob must be split")
+	for i, c := range out {
+		assert.LessOrEqual(t, len(c), maxBytes, "chunk %d over budget: %d bytes", i, len(c))
+	}
+	assert.Equal(t, 1000, strings.Count(strings.Join(out, ""), "x"), "no content lost")
+}
+
 func TestChunkMarkdown_FurtherSplitsOversizedSection(t *testing.T) {
 	// One H2 section with 4 paragraphs of ~80 chars each, limit 100.
 	src := "## big\n\n" +