test: add bats-core test suite for ralph.sh

[Alex-Alaniz]

Summary

Adds the first test suite for Ralph using bats-core, the Bash Automated Testing System.

Why This Matters

Zero tests existed before this PR. Having tests:

• Gives confidence when reviewing PRs
• Documents expected behavior
• Catches regressions before they ship

Test Coverage

Category	Tests
Argument Parsing	--tool amp, --tool claude, --tool= syntax, numeric iterations
Tool Validation	Invalid tool rejection
PRD JSON	Branch name extraction, story structure, required fields
Archive Logic	Branch change detection, last-branch tracking
Progress File	Header format
Completion Detection	<promise>COMPLETE</promise> signal

Running Tests Locally

# Install bats (macOS)
brew install bats-core

# Run tests
bats tests/ralph.bats

Test plan

• Tests pass locally with bats tests/ralph.bats
• Tests are isolated (each test gets fresh temp directory)
• Tests clean up after themselves

Follow-up

CI workflow will be added in a separate PR to automate test runs on push/PR.