feat(hello): add hello module with tests

[jamesrxian]

Summary

• Add src/hello.py with hello(name: str) -> str function that returns "Hello, <name>!"
• Add tests/test_hello.py with 3 unit tests (normal input, empty string, named input)
• Add src/__init__.py to make src a proper importable package

Closes #16

Test plan

• test_hello_world — verifies hello("World") returns "Hello, World!"
• test_hello_empty — verifies hello("") returns "Hello, !"
• test_hello_alice — verifies hello("Alice") returns "Hello, Alice!"

[jamesrxian]

Implementation Summary

Added a hello module (src/hello.py) with a single function hello(name: str) -> str that returns "Hello, <name>!". No external dependencies, no error handling — passes through all inputs per spec. Includes src/__init__.py to make the src package importable.

Integration Test Results

tests/test_hello.py::test_hello_world PASSED
tests/test_hello.py::test_hello_empty PASSED
tests/test_hello.py::test_hello_alice PASSED

14 passed in 0.02s (all existing + new tests)

E2E Test Results

N/A — no e2e tests (skipped per requirements, too simple for agent-level testing).