feat(ingestion): chunk markdown before embedding (#38)

Long markdown files (>~8KB) silently failed to embed because nomic-embed-text
on iguana has a 2048-token context. embed sync logged errors=1 every cycle
with no useful body until #37 added per-item logging — three files exceed
the ceiling: finbert source (8 KB), koala-machine-state (7.1 KB),
litellm-absorption (8.8 KB). Curated knowledge entries should never be
vector-blind.

Approach: chunk-before-embed, no schema change.

vectorstore/chunk.go (new)
- ChunkMarkdown splits at H1/H2 boundaries; sections over maxBytes are
  further split at paragraph boundaries, packing greedily under budget.
- NumberChunks assigns "<parent>#NNNN" storage paths (1-based, zero-padded
  to 4 digits — handles files with up to ~10k sections in stable sort order).
- ParentPath strips the chunk suffix for retrieval-side dedup.

vectorstore/sync.go
- After ChunkMarkdown produces N pieces, each is embedded + upserted as a
  separate brain_embeddings row at "<parent>#NNNN". maxChunkBytes = 4000
  (≈1000 nomic tokens, well under the 2048 ceiling with headroom for
  unicode/code blocks).
- "Already embedded?" check now reduces known paths to parent set via
  ParentPath, so the first chunk hit short-circuits the file.
- Delete walk also reduces via ParentPath; when a parent file disappears,
  every chunk row (and any pre-existing bare-path row, for backward
  compatibility with rows written before this change) gets dropped.

search/search.go
- hybridMerge collapses chunk-path vector hits to parent via ParentPath
  before scope check, RRF accumulation, and hydration. A file with three
  chunk hits returns one result row, not three.

Backward compatibility: pre-existing bare-path rows in brain_embeddings
keep working — ParentPath returns them unchanged, knownParents handles
them as if they were "wiki/foo.md#NNNN" hits, sync skips re-embed, and
search dedup is a no-op for them. No migration required to ship.

Tests:
- chunk_test.go covers short / heading split / oversized section /
  content preservation / chunk numbering / parent-path stripping.
- sync_test.go adds long-file chunking, single-chunk-row short file,
  skip-if-any-chunk-known, delete-all-chunks-of-disappeared-file.
  Existing tests updated for #NNNN paths.
- search_test.go adds chunk-paths-dedupe-to-parent.

Closes gitea/mathias/infra#38.

ingestion/internal/search/search.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/vectorstore"
 )
 
 // VectorSearcher returns the top-limit nearest paths by cosine
@@ -186,17 +187,21 @@ func hybridMerge(ctx context.Context, brainDir string, opts QueryOptions, bm25 [
 		byPath[r.Path] = r
 	}
 	for rank, h := range hits {
-		if opts.Wing != "" && !pathInScope(h.Path, opts.Wing, opts.Hall) {
+		// Vector store keys are chunk paths ("wiki/foo.md#0001"); collapse
+		// back to the parent so multiple chunk hits from the same file
+		// score against a single result row.
+		parent := vectorstore.ParentPath(h.Path)
+		if opts.Wing != "" && !pathInScope(parent, opts.Wing, opts.Hall) {
 			continue
 		}
-		rrf[h.Path] += 1.0 / (rrfK + float64(rank+1))
-		if _, seen := byPath[h.Path]; !seen {
-			r, err := hydrate(brainDir, h.Path)
+		rrf[parent] += 1.0 / (rrfK + float64(rank+1))
+		if _, seen := byPath[parent]; !seen {
+			r, err := hydrate(brainDir, parent)
 			if err != nil {
-				slog.Warn("search: hydrate failed for vector hit", "path", h.Path, "err", err)
+				slog.Warn("search: hydrate failed for vector hit", "path", parent, "err", err)
 				continue
 			}
-			byPath[h.Path] = r
+			byPath[parent] = r
 		}
 	}