feat: cog_categories

Discovery verb over the summary_categories view, grouped one row per
(category, subtype). Parallels cog_gov_search: analysts use it to
find the valid `category` values to pass into cog_spending(),
cog_revenue(), cog_geographic_rollup().

Columns: category, category_type, subtype, n_codes, item_codes
(comma-separated, alphabetical). Optional filters:

  type    = NULL | "spending" | "revenue"
  pattern = regex matched case-insensitively on category

The user-facing "spending" alias is translated internally to the
corpus-native "expenditure" so callers don't have to learn Census
vocabulary, while the returned category_type column preserves the
native value for auditability.

Also: fix @noRd placement in session.R so devtools::document() stops
warning.

Tests: +16 new / 181 total pass. check 0E/0W/0N.

man/cog_categories.Rd

@@ -0,0 +1,28 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/categories.R
+\name{cog_categories}
+\alias{cog_categories}
+\title{List available spending / revenue categories}
+\usage{
+cog_categories(type = NULL, pattern = NULL)
+}
+\arguments{
+\item{type}{Either `NULL` (default, return both spending and revenue
+rows), `"spending"`, or `"revenue"`.}
+
+\item{pattern}{Optional regex matched case-insensitively against the
+`category` column (e.g. `"Police"` or `"Tax"`).}
+}
+\value{
+Tibble with columns `category`, `category_type`, `subtype`,
+  `n_codes`, `item_codes` (comma-separated, alphabetical). Sorted by
+  `category_type`, `category`, `subtype`.
+}
+\description{
+Returns the category taxonomy exposed by the corpus's
+`summary_categories` view, grouped to one row per
+`(category, subtype)` pair. Use this to discover valid `category`
+values for [cog_spending()] / [cog_revenue()] /
+[cog_geographic_rollup()] and to audit which Census item codes feed
+each category.
+}