Replace architecture PNG with Mermaid diagram, remove old flowchart

Author: ToastCoder

README.md

@@ -2,9 +2,31 @@
 
 HR Buddy is an application which is inspired by HR Chatbot portals. Uses combination of Llama (LLM) + Nomic (Embedding) models. Uses Retrieval Augumented Generation for identify the context strictly from the HR Policies PDF.
 
-The whole architecture is explain with the below picture.
-
-![Architecture](media/HRBuddy-Flowchart.png)
+```mermaid
+flowchart TD
+    UI["Streamlit UI"]
+    SESS["Session State<br/>(history, session_id)"]
+    MDB[("MongoDB<br/>Auth & Chat History")]
+
+    subgraph RAG [RAG Engine]
+        direction LR
+        subgraph Hybrid [Hybrid Retrieval]
+            SEM["Semantic Search<br/>ChromaDB + OllamaEmbeddings<br/>(MMR, fetch_k=18)"]
+            BM25["Keyword Search<br/>BM25 (heading-enriched<br/>chunks, top_k=6)"]
+            FUSION["Weighted Fusion<br/>(semantic=0.7, bm25=0.3)"]
+            SEM --> FUSION
+            BM25 --> FUSION
+        end
+        CTX["Context Assembly<br/>(top_k=6 documents)"]
+        LLM["Ollama Llama 3.2<br/>3B params<br/>(temp=0.1, ctx=4096)"]
+        Hybrid --> CTX --> LLM
+    end
+
+    UI --> SESS
+    SESS --> MDB
+    UI -->|"user input + history"| RAG
+    LLM -->|"response stream"| UI
+```
 
 ## Prerequisites