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/mcp/handlers.go

@@ -4,11 +4,13 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"log/slog"
 	"path/filepath"
 	"strings"
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/extract"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
@@ -24,6 +26,10 @@ func (s *Server) tools() []map[string]any {
 	int_ := func(desc string) map[string]any {
 		return map[string]any{"type": "integer", "description": desc}
 	}
+	enum := func(desc string, vals ...string) map[string]any {
+		return map[string]any{"type": "string", "description": desc, "enum": vals}
+	}
+	halls := []string{"facts", "decisions", "failures", "hypotheses", "sources"}
 	schema := func(required []string, props map[string]any) json.RawMessage {
 		b, _ := json.Marshal(map[string]any{
 			"type": "object", "required": required, "properties": props,
@@ -34,20 +40,31 @@ func (s *Server) tools() []map[string]any {
 	return []map[string]any{
 		{
 			"name":        "brain_query",
-			"description": "BM25 full-text search across brain/knowledge/ and brain/wiki/ markdown files.",
+			"description": "BM25 full-text search across brain/knowledge/ and brain/wiki/ markdown files. Optionally scope by wing (topic domain) and hall (memory type).",
 			"inputSchema": schema([]string{"query"}, map[string]any{
 				"query": str("search terms"),
 				"limit": int_("max results, default 5"),
+				"wing":  str("optional wing to scope to, e.g. jepa-fx"),
+				"hall":  enum("optional hall to scope to (requires wing)", halls...),
 			}),
 		},
 		{
 			"name":        "brain_write",
-			"description": "Write a raw knowledge note to brain/knowledge/.",
+			"description": "Write a markdown note to the brain. With wing+hall set, routes to brain/wiki/<wing>/<hall>/ with wing/hall/created_at frontmatter; otherwise writes to brain/knowledge/ (legacy).",
 			"inputSchema": schema([]string{"content"}, map[string]any{
 				"content":  str("markdown content"),
-				"filename": str("optional filename"),
-				"type":     str("optional frontmatter type"),
-				"domain":   str("optional frontmatter domain"),
+				"filename": str("optional filename or slug"),
+				"type":     str("optional frontmatter type (legacy)"),
+				"domain":   str("optional frontmatter domain (legacy)"),
+				"wing":     str("optional topic domain, e.g. jepa-fx"),
+				"hall":     enum("optional memory type (requires wing)", halls...),
+			}),
+		},
+		{
+			"name":        "brain_index",
+			"description": "Regenerate _index.md (Map of Content) for one or all wings under brain/wiki/. Auto-called after brain_write with wing+hall.",
+			"inputSchema": schema([]string{}, map[string]any{
+				"wing": str("optional wing to index; if absent, rebuilds every wing"),
 			}),
 		},
 		{
@@ -104,6 +121,8 @@ func (s *Server) tools() []map[string]any {
 type brainQueryArgs struct {
 	Query string `json:"query"`
 	Limit int    `json:"limit,omitempty"`
+	Wing  string `json:"wing,omitempty"`
+	Hall  string `json:"hall,omitempty"`
 }
 
 func (s *Server) brainQuery(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
@@ -117,7 +136,12 @@ func (s *Server) brainQuery(ctx context.Context, args json.RawMessage) (json.Raw
 	if a.Limit == 0 {
 		a.Limit = 5
 	}
-	results, err := search.Query(s.brainDir, a.Query, a.Limit)
+	results, err := search.Query(s.brainDir, search.QueryOptions{
+		Query: a.Query,
+		Limit: a.Limit,
+		Wing:  a.Wing,
+		Hall:  a.Hall,
+	})
 	if err != nil {
 		return nil, fmt.Errorf("search: %w", err)
 	}
@@ -129,6 +153,8 @@ type brainWriteArgs struct {
 	Filename string `json:"filename,omitempty"`
 	Type     string `json:"type,omitempty"`
 	Domain   string `json:"domain,omitempty"`
+	Wing     string `json:"wing,omitempty"`
+	Hall     string `json:"hall,omitempty"`
 }
 
 func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
@@ -136,13 +162,51 @@ func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.Raw
 	if err := json.Unmarshal(args, &a); err != nil {
 		return nil, fmt.Errorf("parse args: %w", err)
 	}
-	relPath, err := api.WriteNote(s.brainDir, a.Content, a.Filename, a.Type, a.Domain)
+	relPath, err := api.WriteNote(s.brainDir, api.WriteNoteOptions{
+		Content:  a.Content,
+		Filename: a.Filename,
+		Type:     a.Type,
+		Domain:   a.Domain,
+		Wing:     a.Wing,
+		Hall:     a.Hall,
+	})
 	if err != nil {
 		return nil, err
 	}
+	// Auto-regenerate the wing _index.md when the write landed in the
+	// structured wiki. A failure here is best-effort — log and move on,
+	// since the note itself is already written.
+	if a.Wing != "" && a.Hall != "" {
+		if err := brain.BuildWingIndex(s.brainDir, a.Wing); err != nil {
+			slog.Warn("brain_write: auto-index failed", "wing", a.Wing, "err", err)
+		}
+	}
 	return json.Marshal(map[string]string{"path": relPath})
 }
 
+type brainIndexArgs struct {
+	Wing string `json:"wing,omitempty"`
+}
+
+func (s *Server) brainIndex(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
+	var a brainIndexArgs
+	if len(args) > 0 {
+		if err := json.Unmarshal(args, &a); err != nil {
+			return nil, fmt.Errorf("parse args: %w", err)
+		}
+	}
+	if a.Wing == "" {
+		if err := brain.BuildAllWingIndexes(s.brainDir); err != nil {
+			return nil, fmt.Errorf("index: %w", err)
+		}
+		return json.Marshal(map[string]any{"status": "ok", "scope": "all"})
+	}
+	if err := brain.BuildWingIndex(s.brainDir, a.Wing); err != nil {
+		return nil, fmt.Errorf("index: %w", err)
+	}
+	return json.Marshal(map[string]any{"status": "ok", "scope": a.Wing})
+}
+
 type brainIngestRawArgs struct {
 	Source string             `json:"source"`
 	Pages  []pipeline.RawPage `json:"pages"`