feat: package skeleton — DESCRIPTION, NAMESPACE, session/manifest/cache/views

Minimal skeleton for uscogdata v0.1. Internal session layer with lazy
cog_open(), manifest fetch+validate+cache, view registration placeholder.
Depends on DuckDB >=1.0, httr2, jsonlite. inst/schemas/provenance-v1.json
ships the structured provenance JSON Schema.

README.md

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+# uscogdata
+
+Curated R reader for the Civilytics US Census of Governments finance corpus.
+
+Provides unit-level financial profiles, geographic rollups, and peer comparisons
+with auditable provenance and built-in cross-vintage correctness. Reads the
+published corpus (Hive-partitioned parquet + manifest.json) directly from
+Nextcloud via DuckDB httpfs — no local bulk downloads required.
+
+## Status
+
+Under active development (Phase 2 of the cog_pipeline project). See
+`../cog_pipeline/docs/reader-specification.md` for the reader contract this
+package implements.
+
+## Installation
+
+```r
+# pak::pkg_install("gitea.civilytics.org/Civilytics/uscogdata")
+```
+
+## Configuration
+
+- `USCOGDATA_URL` — corpus root URL (public Nextcloud share, trailing slash)
+- `USCOGDATA_CACHE_DIR` — optional override for the manifest cache directory
+- `USCOGDATA_MANIFEST_TTL_SECS` — optional manifest re-fetch TTL (default 3600)