feat: cog_gov_search + cog_mirror + scope-aware verb behavior

Three pieces:

1. cog_gov_search: name/state/type search over canonical_fips_xwalk
   for resolving human-readable place names into canonical_govids.
   Accepts USPS abbrev ('FL') or FIPS int (12) for state; integer
   0-3 or name ('state','county','city','township') for type. Types
   4/5 emit an explanatory cli message and return an empty tibble
   (v0.1 corpus excludes them). USPS<->FIPS table hardcoded with
   50 states + DC + territories; FIPS 66 = GU (not GA).

2. cog_mirror: downloads manifest-listed files to a local directory
   with SHA-256 idempotency (files with matching hash return status
   'cached'). Supports HTTP and local-path fixture URLs. Round-trip
   test: mirror + re-open against the mirror + query Broward 2020
   returns identical results.

3. Scope-aware verbs: .check_govids_in_scope() helper in session.R
   queries canonical_fips_xwalk for the requested govids, emits a
   cli_inform listing any missing ones, and records the found/missing
   sets under provenance$scope. Wired into cog_spending (and
   transitively into cog_revenue, cog_geographic_rollup,
   cog_peer_compare via their cog_spending calls).

Also: dropped dbplyr from Imports (unused).

Tests: +29 (22 search + 12 mirror - 5 refactored) / 159 total pass.
devtools::check() now clean: 0E / 0W / 0N.

R/spending.R

@@ -49,6 +49,7 @@ cog_spending <- function(govid, years, category = NULL,
   if (!is.null(adjust_to_year)) adjust_to_year <- as.integer(adjust_to_year)
 
   con <- .ensure_session()
+  scope <- .check_govids_in_scope(govid)
 
   sql <- .build_verb_sql(view, subtype_col, govid, years, category)
   result <- tibble::as_tibble(DBI::dbGetQuery(con, sql))
@@ -60,7 +61,7 @@ cog_spending <- function(govid, years, category = NULL,
 
   result$notes <- .notes_column(result)
 
-  attr(result, "provenance") <- .build_provenance(
+  prov <- .build_provenance(
     verb           = verb,
     call           = call,
     govid          = govid,
@@ -72,6 +73,9 @@ cog_spending <- function(govid, years, category = NULL,
     sql            = sql,
     subtype_col    = subtype_col
   )
+  prov$scope$govids_found   <- scope$found
+  prov$scope$govids_missing <- scope$missing
+  attr(result, "provenance") <- prov
   result
 }