feat(greet): add greet module with greeting function

[jamesrxian]

Summary

Add a greet module to the testbed package providing a greet(name: str) -> str function that returns "Hello, {name}!", following the existing hello module pattern.

• New file: src/testbed/greet.py — greet function
• New file: tests/test_greet.py — 3 unit tests (importable, basic greeting, empty string)
• All 17 tests passing

Closes #20

[jamesrxian]

Implementation Summary

Added a greet(name: str) -> str function to the testbed package at src/testbed/greet.py that returns f"Hello, {name}!". The module follows the exact pattern of the existing hello module. Unit tests in tests/test_greet.py cover importability, basic greeting, and empty string input. Code review passed with zero issues across all dimensions (spec compliance, quality, correctness, performance).

Integration Test Results

N/A — no integration tests (standalone pure function with no dependencies)

E2E Test Results

N/A — no e2e tests (unit tests sufficient for this simple feature)