feat(vault): tree sidebar with lazy-load folder hierarchy

Replace flat paginated document list with collapsible tree view.
Backend: two-query-per-level approach (files + DISTINCT deeper paths
for virtual folder derivation), path parsing in Go for PG+SQLite
compatibility, text_pattern_ops index for prefix range scans.
Frontend: VaultTree component with doc_type colored icons, compact
single-line nodes, truncate-middle filenames, scope dots, hover
tooltip for date. Sidebar expanded to 384px on large screens.

Also fixes pre-existing SQLite scanVaultDocRow missing path_basename.

internal/store/sqlitestore/schema.sql

@@ -1542,6 +1542,7 @@ CREATE INDEX IF NOT EXISTS idx_vault_docs_type ON vault_documents(agent_id, doc_
 CREATE INDEX IF NOT EXISTS idx_vault_docs_hash ON vault_documents(content_hash);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_team ON vault_documents(team_id);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_basename ON vault_documents(tenant_id, path_basename);
+CREATE INDEX IF NOT EXISTS idx_vault_docs_path_prefix ON vault_documents(tenant_id, path);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_delegation
     ON vault_documents(json_extract(metadata, '$.delegation_id'))
     WHERE json_extract(metadata, '$.delegation_id') IS NOT NULL;