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/path.go

@@ -0,0 +1,70 @@
+// Package brain provides the wing/hall path taxonomy used by the brain
+// wiki layout. A note's canonical location is
+// brain/wiki/<wing>/<hall>/<slug>.md, where Wing is a free-form topic
+// domain and Hall is one of a closed vocabulary of memory types.
+package brain
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// ValidHalls is the closed vocabulary of hall names. A hall captures the
+// memory type of a note within any wing.
+var ValidHalls = map[string]bool{
+	"facts":      true,
+	"decisions":  true,
+	"failures":   true,
+	"hypotheses": true,
+	"sources":    true,
+}
+
+// IsValidHall reports whether h is in the closed Hall vocabulary.
+func IsValidHall(h string) bool {
+	return ValidHalls[h]
+}
+
+// NotePath resolves the canonical filesystem path for a note given a
+// wing, hall, and slug. Returns an error if hall is not in ValidHalls
+// or if wing/slug sanitise to empty strings.
+//
+// The returned path is brain/wiki/<wing>/<hall>/<slug>.md with all
+// segments sanitised: lowercased, alphanumerics and hyphens only.
+func NotePath(brainDir, wing, hall, slug string) (string, error) {
+	if !IsValidHall(hall) {
+		return "", fmt.Errorf("invalid hall %q: must be one of facts/decisions/failures/hypotheses/sources", hall)
+	}
+	w := Sanitise(wing)
+	if w == "" {
+		return "", fmt.Errorf("invalid wing %q: must contain at least one alphanumeric character", wing)
+	}
+	s := Sanitise(strings.TrimSuffix(slug, ".md"))
+	if s == "" {
+		return "", fmt.Errorf("invalid slug %q: must contain at least one alphanumeric character", slug)
+	}
+	return filepath.Join(brainDir, "wiki", w, hall, s+".md"), nil
+}
+
+// Sanitise lowercases s and keeps only [a-z0-9-], collapsing any other
+// character (including path separators) to a hyphen. Leading/trailing
+// hyphens and runs of hyphens are collapsed.
+func Sanitise(s string) string {
+	s = strings.ToLower(strings.TrimSpace(s))
+	var b strings.Builder
+	prevHyphen := true
+	for _, r := range s {
+		switch {
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
+			b.WriteRune(r)
+			prevHyphen = false
+		case r == '-' || r == '_' || r == ' ' || r == '/' || r == '\\' || r == '.':
+			if !prevHyphen {
+				b.WriteByte('-')
+				prevHyphen = true
+			}
+		}
+	}
+	out := b.String()
+	return strings.Trim(out, "-")
+}