feat(brain): structured wing/hall taxonomy + obsidian-compatible layout

Adds a two-dimensional address (wing, hall) to brain notes. A wing is a
topic domain (e.g. jepa-fx, hyperguild); a hall is one of a closed
vocabulary of memory types (facts, decisions, failures, hypotheses,
sources). Notes route to brain/wiki/<wing>/<hall>/<slug>.md with
wing/hall/created_at YAML frontmatter, making the directory a valid
Obsidian vault.

Changes:
- new package ingestion/internal/brain (NotePath, ValidHalls, Sanitise,
  BuildWingIndex, BuildAllWingIndexes)
- api.WriteNote refactored to WriteNoteOptions; wing+hall routes to
  brain/wiki/, otherwise falls back to brain/knowledge/ (legacy)
- search.Query → QueryOptions with optional Wing/Hall filtering; Result
  carries wing/hall extracted from frontmatter or path segments
- MCP tools brain_write and brain_query gain optional wing/hall params
  (hall enum-validated); new brain_index tool regenerates _index.md MOC
- POST /index REST endpoint mirrors brain_index
- brain_write auto-rebuilds the wing's _index.md after a wing+hall write
- scripts/migrate-brain-halls.sh migrates flat brain/wiki/{concepts,entities}/
  into the new layout (dry-run by default, --commit applies)

All existing tests pass; new tests cover wing/hall write routing, scope
filtering, invalid hall rejection, _index.md generation, and migration
script paths.

Closes hyperguild#1.

ingestion/internal/search/search.go

@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
 )
 
 // Result is a single search hit from the brain wiki.
@@ -17,24 +19,41 @@ type Result struct {
 	Title   string `json:"title"`
 	Excerpt string `json:"excerpt"`
 	Score   int    `json:"score"`
+	Wing    string `json:"wing,omitempty"`
+	Hall    string `json:"hall,omitempty"`
 }
 
-// Query searches all .md files under brainDir/wiki/ for pages containing
-// any of the whitespace-separated terms in query. Returns up to limit results
-// sorted by score descending.
-func Query(brainDir, query string, limit int) ([]Result, error) {
-	if limit <= 0 {
-		limit = 5
+// QueryOptions configures a search.
+//
+// When Wing is set, the walk is restricted to brain/wiki/<wing>/.
+// When Hall is additionally set, the walk is restricted to
+// brain/wiki/<wing>/<hall>/. Without either, the legacy walk over
+// brain/knowledge/ and brain/wiki/ is used.
+type QueryOptions struct {
+	Query string
+	Limit int
+	Wing  string
+	Hall  string
+}
+
+// Query searches the brain. Returns up to opts.Limit results sorted by
+// score descending. Empty query returns nil.
+func Query(brainDir string, opts QueryOptions) ([]Result, error) {
+	if opts.Limit <= 0 {
+		opts.Limit = 5
 	}
-	terms := strings.Fields(strings.ToLower(query))
+	terms := strings.Fields(strings.ToLower(opts.Query))
 	if len(terms) == 0 {
 		return nil, nil
 	}
 
-	var results []Result
+	roots, err := resolveRoots(brainDir, opts.Wing, opts.Hall)
+	if err != nil {
+		return nil, err
+	}
 
-	for _, subdir := range []string{"knowledge", "wiki"} {
-		dir := filepath.Join(brainDir, subdir)
+	var results []Result
+	for _, dir := range roots {
 		if _, statErr := os.Stat(dir); os.IsNotExist(statErr) {
 			continue
 		}
@@ -46,13 +65,11 @@ func Query(brainDir, query string, limit int) ([]Result, error) {
 			if d.IsDir() || !strings.HasSuffix(path, ".md") {
 				return nil
 			}
-
 			content, err := os.ReadFile(path)
 			if err != nil {
 				slog.Warn("search: skipping unreadable file", "path", path, "err", err)
 				return nil
 			}
-
 			lower := strings.ToLower(string(content))
 			score := 0
 			for _, term := range terms {
@@ -61,18 +78,19 @@ func Query(brainDir, query string, limit int) ([]Result, error) {
 			if score == 0 {
 				return nil
 			}
-
 			rel, err := filepath.Rel(brainDir, path)
 			if err != nil {
 				return fmt.Errorf("rel path: %w", err)
 			}
 			rel = filepath.ToSlash(rel)
-
+			wing, hall := extractWingHall(string(content), rel)
 			results = append(results, Result{
 				Path:    rel,
 				Title:   extractTitle(string(content), d.Name()),
 				Excerpt: excerpt(string(content), 300),
 				Score:   score,
+				Wing:    wing,
+				Hall:    hall,
 			})
 			return nil
 		})
@@ -84,12 +102,81 @@ func Query(brainDir, query string, limit int) ([]Result, error) {
 	sort.Slice(results, func(i, j int) bool {
 		return results[i].Score > results[j].Score
 	})
-	if len(results) > limit {
-		results = results[:limit]
+	if len(results) > opts.Limit {
+		results = results[:opts.Limit]
 	}
 	return results, nil
 }
 
+// resolveRoots returns the directories to walk for the given wing/hall
+// filters. Validates hall against the closed vocabulary when set.
+func resolveRoots(brainDir, wing, hall string) ([]string, error) {
+	if hall != "" && !brain.IsValidHall(hall) {
+		return nil, fmt.Errorf("invalid hall %q", hall)
+	}
+	if wing != "" {
+		w := brain.Sanitise(wing)
+		if w == "" {
+			return nil, fmt.Errorf("invalid wing %q", wing)
+		}
+		if hall != "" {
+			return []string{filepath.Join(brainDir, "wiki", w, hall)}, nil
+		}
+		return []string{filepath.Join(brainDir, "wiki", w)}, nil
+	}
+	if hall != "" {
+		return nil, fmt.Errorf("hall filter requires wing")
+	}
+	return []string{
+		filepath.Join(brainDir, "knowledge"),
+		filepath.Join(brainDir, "wiki"),
+	}, nil
+}
+
+// extractWingHall reads wing/hall from frontmatter first, falling back to
+// path segments brain/wiki/<wing>/<hall>/.
+func extractWingHall(content, relPath string) (wing, hall string) {
+	scanner := bufio.NewScanner(strings.NewReader(content))
+	inFrontmatter := false
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.TrimSpace(line) == "---" {
+			if !inFrontmatter {
+				inFrontmatter = true
+				continue
+			}
+			break
+		}
+		if !inFrontmatter {
+			continue
+		}
+		key, val, ok := strings.Cut(line, ":")
+		if !ok {
+			continue
+		}
+		v := strings.Trim(strings.TrimSpace(val), `"'`)
+		switch strings.TrimSpace(key) {
+		case "wing":
+			wing = v
+		case "hall":
+			hall = v
+		}
+	}
+	if wing != "" && hall != "" {
+		return wing, hall
+	}
+	parts := strings.Split(relPath, "/")
+	if len(parts) >= 4 && parts[0] == "wiki" {
+		if wing == "" {
+			wing = parts[1]
+		}
+		if hall == "" && brain.IsValidHall(parts[2]) {
+			hall = parts[2]
+		}
+	}
+	return wing, hall
+}
+
 func extractTitle(content, filename string) string {
 	scanner := bufio.NewScanner(strings.NewReader(content))
 	inFrontmatter := false
@@ -113,7 +200,6 @@ func extractTitle(content, filename string) string {
 }
 
 func excerpt(content string, maxLen int) string {
-	// Skip frontmatter, return first maxLen chars of body.
 	parts := strings.SplitN(content, "---", 3)
 	body := content
 	if len(parts) == 3 {