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.

NAMESPACE

@@ -3,6 +3,8 @@
 export(cog_explain)
 export(cog_find_peers)
 export(cog_geographic_rollup)
+export(cog_gov_search)
+export(cog_mirror)
 export(cog_peer_compare)
 export(cog_revenue)
 export(cog_spending)