structure: reify-orphans endpoint + "give these a home" dialog

Turns the four synthetic tree containers into a real migration path.
Clicking any of them ("My Workspace", "Teams", "Direct",
"Ungrouped") opens a dialog that creates a real
org → company → team chain and re-parents every orphan into it, all
in one DB transaction.

cm-db (new module structure_reify)
- orphan_agents / orphan_teams / orphan_companies: workspace-scoped
  SELECTs of entities without a parent binding in team_members /
  company_teams / org_companies. Used both by the dialog's counter
  and internally by the migration.
- count_orphans: cheap combined-count via three subqueries in a
  single SELECT so the dialog only round-trips once for the header.
- reify_orphans(pool, ws, org_name, company_name, team_name):
    1. begins a tx
    2. inserts a new org + company + team (all `flat`, empty graphs
       — user can shape them later via the existing PATCH endpoints)
    3. binds company under org (org_companies "n0")
    4. binds team under company (company_teams "n0")
    5. inserts team_members rows for every orphan agent (n1, n2, …)
    6. inserts company_teams rows for every orphan team
    7. inserts org_companies rows for every orphan company
    8. commits, returns the created ids + moved counts

cm-api (routes/structure)
- GET /api/structure/orphan-counts → { agents, teams, companies }
- POST /api/structure/reify-orphans → { org_id, company_id, team_id,
  moved_* }. Trims + rejects any empty name; validates before
  starting the transaction so a 400 never rolls anything back.

Frontend
- New OrphanMigrationDialog: fetches counts on open, three name
  fields (defaults: Organization "My Workspace", Company "General",
  Team "Everyone"), POSTs on save. "Nothing to migrate" state
  disables the save button when the workspace is already fully
  wired. Copy explicitly notes that everything is renameable in the
  sidebar afterward.
- Dashboard: onTreeSelect now branches on SYNTHETIC_TREE_IDS —
  clicking a synthetic node opens the dialog instead of falling
  through to the (nonexistent) selection. On successful reify,
  router.refresh() so the sidebar + world viz reflect the new real
  chain.

What this doesn't do yet (next commit)
- Wizard auto-materialize: when creating a team/company via wizard,
  auto-create parent placeholders if they don't exist. Deferred so
  this commit stays focused.

.sqlx/query-55c02f024dbee63480d187f171cc0a61a2d176c2090b67068ce53c8e9f8c35af.json

@@ -0,0 +1,22 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT a.id AS id\n           FROM agents a\n          WHERE a.workspace_id = $1\n            AND a.deleted_at IS NULL\n            AND NOT EXISTS (SELECT 1 FROM team_members tm WHERE tm.claw_id = a.id)",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "id",
+        "type_info": "Uuid"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Uuid"
+      ]
+    },
+    "nullable": [
+      false
+    ]
+  },
+  "hash": "55c02f024dbee63480d187f171cc0a61a2d176c2090b67068ce53c8e9f8c35af"
+}