docs: add talk-to-data case study

Add anonymized architecture, governance examples, diagrams, and interview materials.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

architecture_dag.md

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+# Quotation Semantic Layer - Data Flow DAG
+# SAP Source -> Snowflake -> dbt Transforms -> Semantic Views -> Cortex -> Chatbot Output
+
+graph LR
+    SAP["🗄️ SAP ERP System<br/>(Sales & Discount Data)"]
+    
+    EXT["📥 Snowflake<br/>External Stage"]
+    
+    RAW["raw.quotations<br/>raw.discount_conditions"]
+    
+    STG["STAGING Layer<br/>stg_quotes<br/>stg_discount_conditions<br/>stg_product_master"]
+    
+    TRF["TRANSFORM Layer<br/>trf_quotation<br/>trf_chatbot_quotation"]
+    
+    DIM["Reference Dimensions<br/>dim_customer_discount_policy<br/>dim_product_portfolio<br/>fct_agreement_discounts"]
+    
+    SEMA["SEMANTIC LAYER<br/>sv_quotation<br/>sv_chatbot_quotation"]
+    
+    CORTEX["Cortex Analytics<br/>Guardrails & Information Protection<br/>Access Control<br/>Freshness Contract"]
+    
+    UI["Snowflake UI<br/>(BI Dashboard)"]
+    CHAT["🤖 Internal Workspace<br/>(Chatbot)"]
+    
+    SAP -->|Daily Batch 05:00 UTC| EXT
+    EXT -->|Load| RAW
+    
+    RAW -->|Column mapping<br/>Type casting| STG
+    
+    STG -->|Join dimensions<br/>Calculate discounts<br/>Apply governance flags| TRF
+    DIM -->|Enrich with policies| TRF
+    
+    TRF -->|Expose semantic<br/>business logic<br/>Hide implementation| SEMA
+    
+    SEMA -->|Apply guardrails<br/>Protect sensitive information<br/>Enforce freshness| CORTEX
+    
+    CORTEX -->|Query & Visualize| UI
+    CORTEX -->|LLM consumption<br/>Prompt engineering layer| CHAT
+    
+    CHAT -->|<1 min<br/>Quote Answers| USER["👤 Sales & Ops<br/>Team"]
+    UI -->|Dashboard<br/>Reporting| OPS["📊 Operations<br/>Team"]
+    
+    SEMA -.->|Data Contract<br/>Lineage| AUDIT["🔒 Audit & Compliance<br/>- Access logs<br/>- Policy changes<br/>- Exception escalations"]
+    
+    style SAP fill:#d4a574
+    style CORTEX fill:#ff9999
+    style SEMA fill:#99ccff
+    style CHAT fill:#99ff99
+    style USER fill:#ffcc99
+    style AUDIT fill:#cc99ff
+
+classDef layer_raw fill:#e1e4e8
+classDef layer_stg fill:#d0d7de
+classDef layer_trf fill:#adb8c1
+classDef layer_sema fill:#6e7681
+
+class RAW layer_raw
+class STG layer_stg
+class TRF layer_trf
+class SEMA layer_sema