TICKET-037: Add edge case validation to CLI and workflow

[belarusian]

TICKET-037: Add edge case validation to CLI and workflow

Module: alloc/cli.py, alloc/utils/workflow.py
Priority: Medium — improve robustness

What to Implement

Add input validation:

1. Empty tickers list → raise ValueError
2. Zero/negative positions → raise ValueError
3. Invalid JSON positions → meaningful error message
4. Empty workflow result → graceful handling

Verification

• pytest tests/ -x -q passes
• ruff check alloc/ passes
• mypy alloc/ --ignore-missing-imports passes