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.

scripts/migrate-brain-halls.sh

@@ -0,0 +1,135 @@
+#!/usr/bin/env bash
+# migrate-brain-halls.sh — move flat brain/wiki/{concepts,entities}/ notes
+# into the structured brain/wiki/<wing>/<hall>/ layout introduced by
+# hyperguild#1.
+#
+# Reads each note's YAML frontmatter:
+#   type:   maps to hall (decision, hypothesis, failure, source → eponymous;
+#           concept, entity, anything else → facts)
+#   domain: maps to wing (sanitised: lowercase, alphanumerics + hyphens);
+#           empty → "general"
+#
+# Dry-run by default. Pass --commit to actually move files. Idempotent:
+# already-migrated notes (already under a Wing dir) are left alone.
+#
+# Usage:
+#   scripts/migrate-brain-halls.sh /path/to/brain            # dry-run
+#   scripts/migrate-brain-halls.sh --commit /path/to/brain   # apply
+
+set -euo pipefail
+
+COMMIT=0
+BRAIN=""
+for arg in "$@"; do
+  case "$arg" in
+    --commit) COMMIT=1 ;;
+    -h|--help)
+      sed -n '2,18p' "$0"
+      exit 0
+      ;;
+    *) BRAIN="$arg" ;;
+  esac
+done
+
+if [[ -z "$BRAIN" ]]; then
+  echo "error: brain directory required" >&2
+  echo "usage: $0 [--commit] <brain-dir>" >&2
+  exit 2
+fi
+
+if [[ ! -d "$BRAIN" ]]; then
+  echo "error: $BRAIN is not a directory" >&2
+  exit 2
+fi
+
+WIKI="$BRAIN/wiki"
+if [[ ! -d "$WIKI" ]]; then
+  echo "no $WIKI/ — nothing to migrate"
+  exit 0
+fi
+
+sanitise() {
+  # lowercase, replace non-alnum with hyphen, collapse hyphens, trim
+  local s
+  s=$(printf '%s' "$1" | tr '[:upper:]' '[:lower:]' \
+        | sed -E 's/[^a-z0-9]+/-/g; s/^-+//; s/-+$//; s/-+/-/g')
+  printf '%s' "$s"
+}
+
+# extract_frontmatter_value <file> <key>
+# Echoes the value (trimmed, unquoted) of `key:` from a leading YAML
+# frontmatter block. Empty if absent or no frontmatter.
+extract_frontmatter_value() {
+  awk -v key="$2" '
+    BEGIN { in_fm = 0; first = 1 }
+    /^---[[:space:]]*$/ {
+      if (first) { in_fm = 1; first = 0; next }
+      if (in_fm) { exit }
+    }
+    in_fm {
+      idx = index($0, ":")
+      if (idx == 0) next
+      k = substr($0, 1, idx-1)
+      v = substr($0, idx+1)
+      gsub(/^[[:space:]]+|[[:space:]]+$/, "", k)
+      gsub(/^[[:space:]]+|[[:space:]]+$/, "", v)
+      gsub(/^["'\'']|["'\'']$/, "", v)
+      if (k == key) { print v; exit }
+    }
+  ' "$1"
+}
+
+hall_for_type() {
+  case "$1" in
+    decision|decisions)   echo "decisions" ;;
+    hypothesis|hypotheses) echo "hypotheses" ;;
+    failure|failures)     echo "failures" ;;
+    source|sources)       echo "sources" ;;
+    *)                    echo "facts" ;;
+  esac
+}
+
+declare -i moved=0 skipped=0
+
+migrate_source_dir() {
+  local src="$1"
+  [[ -d "$src" ]] || return 0
+  while IFS= read -r -d '' f; do
+    local typ domain wing hall slug dest
+    typ=$(extract_frontmatter_value "$f" type)
+    domain=$(extract_frontmatter_value "$f" domain)
+    hall=$(hall_for_type "$typ")
+    wing=$(sanitise "${domain:-general}")
+    [[ -z "$wing" ]] && wing="general"
+    slug=$(basename "$f" .md)
+    dest="$WIKI/$wing/$hall/$slug.md"
+
+    if [[ "$f" == "$dest" ]]; then
+      skipped=$((skipped + 1))
+      continue
+    fi
+
+    if [[ -e "$dest" ]]; then
+      echo "skip (target exists): $f → $dest"
+      skipped=$((skipped + 1))
+      continue
+    fi
+
+    if [[ "$COMMIT" -eq 1 ]]; then
+      mkdir -p "$(dirname "$dest")"
+      git -C "$BRAIN" mv "$f" "$dest" 2>/dev/null || mv "$f" "$dest"
+    fi
+    echo "move: $f → $dest"
+    moved=$((moved + 1))
+  done < <(find "$src" -maxdepth 1 -type f -name '*.md' -print0)
+}
+
+migrate_source_dir "$WIKI/concepts"
+migrate_source_dir "$WIKI/entities"
+
+echo
+if [[ "$COMMIT" -eq 1 ]]; then
+  echo "moved=$moved skipped=$skipped (committed)"
+else
+  echo "moved=$moved skipped=$skipped (dry-run — pass --commit to apply)"
+fi