Fix: Stop tracking ChromaDB SQLite files to prevent merge conflicts

### Problem Description
Currently, `Backend/db/chromadb/chroma.sqlite3` is tracked in version control. This causes the file to appear as "modified" in `git status` after every backend run, leading to constant merge conflicts for contributors and unnecessary repository bloat.

### Evidence
Every time the backend runs, `Backend/agent/vector_store.py` initializes `chromadb.PersistentClient(path="db/chromadb")`, which modifies the SQLite binary file at runtime.

### Proposed Solution
1. **Stop Tracking:** Run `git rm --cached Backend/db/chromadb/chroma.sqlite3` to remove the file from the remote repo while keeping it locally for current users.
2. **Ignore Future Changes:** Update `Backend/.gitignore` to strictly exclude ChromaDB runtime files:
```gitignore
# ChromaDB
db/chromadb/*.sqlite3
db/chromadb/*.sqlite3-shm
db/chromadb/*.sqlite3-wal
3.Add a **README** note that the DB auto-initializes on first run.

Does the committed chroma.sqlite3 contain required seed data?
if Yes: I will add a Python seeding script.
if No: I will remove the binary file.

Please assign this issue to me, ready to implement.