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>

USAGE.md

@@ -196,7 +196,7 @@ graph LR
 | `dc_unemployment_rate()` | LAUST110000000000003 | DC, not SA |
 | `state_unemployment(fips)` | 4 LAUS series | Rate/employed/unemployed/LF |
 | `dc_region_unemployment()` | 6 series | DC/MD/VA + 3 MSAs |
-| `job_openings_level()` | JTS000000000000JOL | JOLTS openings |
+| `job_openings_level()` | JTS000000000000000JOL | JOLTS openings |
 | `jolts_dashboard()` | 5 series | Full JOLTS flow |
 | `qcew_state(fips)` | EN… | QCEW quarterly by state |