Fix broken series-ID builders; env-var config; project README

Repair every dead series-ID encoding (library now 82/82 query helpers
return live data, verified against the BLS API):

- JOLTS: 21-char format (was 18) — add state/area/sizeclass fields
- OES: national area code 0000000 (was invalid 0000400)
- ECI: correct owner/component/estimate encoding, default unadjusted (CIU)
- Productivity: 4-digit sector + 4-digit measure codes (was 2+3)
- QCEW: 13-char timeseries-API form (ENUUS00010510 / ENU{fips}00010{own}10)
- PPI: repoint finished-goods -> final demand (WPUFD4); keep alias
- ECEC: drop fabricated health-insurance/retirement helpers; add total benefits
- wages SOC: software developers 151132 -> 151252 (2018 SOC)

Tooling/docs:
- config reads BLS_API_KEY env var (takes precedence; config.py gitignored)
- add requirements.txt
- rewrite README as project front door + coverage table + limitations
- correct JOLTS/OES tables in series_id_formats.md, USAGE.md, dataset explorer

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bls_client/queries/prices.py

@@ -66,9 +66,13 @@ def ppi_all_commodities() -> str:
     return ppi_commodity("00000000")
 
 
-def ppi_finished_goods() -> str:
-    """PPI — finished goods."""
-    return ppi_commodity("3")
+def ppi_final_demand() -> str:
+    """PPI — final demand (successor to the discontinued 'finished goods' index)."""
+    return ppi_commodity("FD4")
+
+
+# Backwards-compatible alias; the legacy "finished goods" index was replaced by final demand.
+ppi_finished_goods = ppi_final_demand
 
 
 def ppi_energy() -> str:
@@ -82,7 +86,7 @@ def ppi_food() -> str:
 def ppi_dashboard() -> dict:
     return {
         "All Commodities": ppi_all_commodities(),
-        "Finished Goods":  ppi_finished_goods(),
+        "Final Demand":    ppi_final_demand(),
         "Food":            ppi_food(),
         "Energy":          ppi_energy(),
     }