Migrate Genesys CX Cloud TEI study to the pattern; retire Streamlit app

studies/202512_GenesysCX -> studies/202512_TEI_Genesys_CX_Cloud,
rebuilt as pattern Variant 4 (TEI composite reproduction):

- teicalc/ self-contained engine: Forrester's tables as the never-edited
  verbatim anchor (incl. the p.14 typo note and the $0 AI-token line),
  generic model/scenarios/staging carried over from the Amazon Connect
  study, ClientDrivers overlay (agents / weekly interactions / revenue,
  flat composite so no growth re-base) with ai_tokens_annual as a direct
  input for the token line the published study models at $0
- one deliverable notebook (business_case.ipynb): widget-pair sidebar
  drivers incl. the AI-token price, published-vs-overlay KPI columns,
  cash-flow/waterfall/scenario charts, verification gate, backstage JSON
  data appendix
- gate + tests reproduce the published totals within $2: NPV $10.8M /
  ROI 266% (engine $10,783,466 / 265.79%; payback 3.3 months, not
  headlined in the PDF); 29 study tests, headless nbconvert green,
  stage simulation leak-free, exports carry the appendix
- old Athena workflow (00_provision..04_export, config.py, seed_data.py,
  PALLADIUM_GENESYSCX_* keys, ATHENA_EXPECTED reconciliation) deleted;
  git history preserves it

With the last legacy study migrated, the retirement lands too:
- app/ (Streamlit UI) and core/notebook_helpers deleted; nothing else
  imported them
- streamlit stripped from pyproject extras, requirements.txt, Makefile;
  .env.example reduced to the Athena keys; 00_setup.ipynb and
  core/bootstrap.py repointed at the pattern studies
- root README reworked: self-contained studies + slim core/ Athena
  toolkit (tei_client, calculations, export, cli)

All suites green: Genesys 29, Amazon Connect 27, CTM 55, template 7,
root 58.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.env.example

@@ -2,24 +2,3 @@
 # which prompts for these and writes .env for you.
 ATHENA_BASE_URL=https://athena.ouranos.helu.ca
 ATHENA_API_KEY=your-api-key-here
-
-# Optional — pre-set the active study + tool so notebooks/CLI pick them up
-# without editing config.py. 00_provision.ipynb writes these for you.
-# A TEI tool attaches to exactly ONE of proposal / engagement.
-# PALLADIUM_REPORT_PUBLIC_ID=
-# PALLADIUM_TOOL_PUBLIC_ID=
-# PALLADIUM_PROPOSAL_ID=
-# PALLADIUM_ENGAGEMENT_ID=
-
-# ---------------------------------------------------------------------------
-# Locale / display formatting (Streamlit app)
-# ---------------------------------------------------------------------------
-# Currency symbol prefix (default: $)
-# PALLADIUM_CURRENCY_SYMBOL=$
-#
-# Thousands separator (default: , for Americas/UK; use . for continental Europe)
-# PALLADIUM_THOUSANDS_SEP=,
-#
-# Decimal separator (default: . for Americas/UK; use , for continental Europe)
-# PALLADIUM_DECIMAL_SEP=.
-