chore: retire requirements.txt in favor of pyproject.toml

Addresses code quality suggestion from review: requirements.txt was a
redundant secondary source of truth. Dependencies are now exclusively
managed in pyproject.toml with proper runtime/dev separation.

- requirements.txt: replaced with a comment redirecting to pip install -e '[dev]'
- README.md: updated install command from pip install -r requirements.txt
  to pip install -e '[dev]'

README.md

@@ -19,8 +19,8 @@ A web-based dashboard for tmux sessions. Access and manage all your tmux session
 ## Running
 
 ```bash
-# Install dependencies
-pip install -r requirements.txt
+# Install dependencies (runtime + dev)
+pip install -e ".[dev]"
 
 # Start coordinator (from this directory)
 python -m uvicorn coordinator.main:app --host 0.0.0.0 --port 8099