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/brain/index.go

@@ -0,0 +1,161 @@
+package brain
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+)
+
+// noteEntry is one row in a Wing _index.md.
+type noteEntry struct {
+	Hall    string
+	Slug    string
+	Title   string
+	Created string
+}
+
+// BuildWingIndex regenerates brain/wiki/<wing>/_index.md as a Map of
+// Content listing every note in that wing with its Hall and creation
+// date. Returns nil if the wing directory does not exist.
+func BuildWingIndex(brainDir, wing string) error {
+	w := Sanitise(wing)
+	if w == "" {
+		return fmt.Errorf("invalid wing %q", wing)
+	}
+	wingDir := filepath.Join(brainDir, "wiki", w)
+	if _, err := os.Stat(wingDir); os.IsNotExist(err) {
+		return nil
+	} else if err != nil {
+		return fmt.Errorf("stat wing: %w", err)
+	}
+
+	entries, err := collectWingEntries(wingDir)
+	if err != nil {
+		return err
+	}
+	sort.Slice(entries, func(i, j int) bool {
+		if entries[i].Hall != entries[j].Hall {
+			return entries[i].Hall < entries[j].Hall
+		}
+		return entries[i].Slug < entries[j].Slug
+	})
+
+	var b strings.Builder
+	fmt.Fprintf(&b, "# %s\n\n", w)
+	b.WriteString("| Hall | Note | Created |\n")
+	b.WriteString("|------|------|---------|\n")
+	for _, e := range entries {
+		fmt.Fprintf(&b, "| %s | [%s](%s/%s.md) | %s |\n", e.Hall, e.Title, e.Hall, e.Slug, e.Created)
+	}
+
+	dest := filepath.Join(wingDir, "_index.md")
+	return os.WriteFile(dest, []byte(b.String()), 0o644)
+}
+
+// BuildAllWingIndexes regenerates _index.md for every wing under brain/wiki/.
+func BuildAllWingIndexes(brainDir string) error {
+	wikiDir := filepath.Join(brainDir, "wiki")
+	ents, err := os.ReadDir(wikiDir)
+	if os.IsNotExist(err) {
+		return nil
+	}
+	if err != nil {
+		return fmt.Errorf("read wiki: %w", err)
+	}
+	for _, e := range ents {
+		if !e.IsDir() {
+			continue
+		}
+		if err := BuildWingIndex(brainDir, e.Name()); err != nil {
+			return fmt.Errorf("index %s: %w", e.Name(), err)
+		}
+	}
+	return nil
+}
+
+func collectWingEntries(wingDir string) ([]noteEntry, error) {
+	var out []noteEntry
+	ents, err := os.ReadDir(wingDir)
+	if err != nil {
+		return nil, fmt.Errorf("read wing: %w", err)
+	}
+	for _, hallEnt := range ents {
+		if !hallEnt.IsDir() {
+			continue
+		}
+		hall := hallEnt.Name()
+		if !IsValidHall(hall) {
+			continue
+		}
+		hallDir := filepath.Join(wingDir, hall)
+		notes, err := os.ReadDir(hallDir)
+		if err != nil {
+			return nil, fmt.Errorf("read hall %s: %w", hall, err)
+		}
+		for _, n := range notes {
+			if n.IsDir() || !strings.HasSuffix(n.Name(), ".md") || n.Name() == "_index.md" {
+				continue
+			}
+			slug := strings.TrimSuffix(n.Name(), ".md")
+			full := filepath.Join(hallDir, n.Name())
+			title, created := readTitleAndCreated(full, slug)
+			out = append(out, noteEntry{Hall: hall, Slug: slug, Title: title, Created: created})
+		}
+	}
+	return out, nil
+}
+
+// readTitleAndCreated reads YAML frontmatter for title + created_at; falls
+// back to slug and file mtime when absent.
+func readTitleAndCreated(path, slug string) (string, string) {
+	f, err := os.Open(path)
+	if err != nil {
+		return slug, ""
+	}
+	defer func() { _ = f.Close() }()
+
+	title, created := "", ""
+	scanner := bufio.NewScanner(f)
+	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 "title":
+			title = v
+		case "created_at":
+			if t, err := time.Parse(time.RFC3339, v); err == nil {
+				created = t.UTC().Format("2006-01-02")
+			} else {
+				created = v
+			}
+		}
+	}
+	if title == "" {
+		title = slug
+	}
+	if created == "" {
+		if info, err := os.Stat(path); err == nil {
+			created = info.ModTime().UTC().Format("2006-01-02")
+		}
+	}
+	return title, created
+}