Add CI/CD workflows and security policy

.github/CI_CD_PRIMER.md

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+# CI/CD Primer for Stegasoo
+
+## What is CI/CD?
+
+**CI** = Continuous Integration
+**CD** = Continuous Deployment
+
+Think of it as a robot assistant that automatically:
+1. **Tests your code** every time you push
+2. **Checks formatting** so code stays consistent
+3. **Publishes releases** when you tag a version
+
+```
+You push code → GitHub runs workflows → You get ✓ or ✗
+```
+
+---
+
+## How It Works
+
+### The Trigger
+
+When you `git push` or create a Pull Request, GitHub looks for workflow files in:
+```
+.github/workflows/*.yml
+```
+
+Each `.yml` file defines a workflow - a series of steps to run.
+
+### The Runners
+
+GitHub provides free Linux/Mac/Windows VMs that:
+1. Clone your repo
+2. Set up Python
+3. Run your commands
+4. Report success/failure
+
+You don't manage servers - GitHub does.
+
+---
+
+## Your Workflows
+
+### 1. `test.yml` - Run Tests on Every Push
+
+**When it runs:** Every push, every PR
+
+**What it does:**
+```
+1. Spins up Ubuntu VM
+2. Installs Python 3.10, 3.11, 3.12
+3. Installs your package + dependencies
+4. Runs pytest
+5. Reports pass/fail
+```
+
+**You'll see:** Green ✓ or red ✗ on your commits
+
+### 2. `lint.yml` - Check Code Style
+
+**When it runs:** Every push, every PR
+
+**What it does:**
+```
+1. Runs ruff (fast Python linter)
+2. Checks black formatting
+3. Fails if code isn't formatted
+```
+
+**Why:** Keeps code consistent, catches common bugs
+
+### 3. `release.yml` - Publish to PyPI
+
+**When it runs:** Only when you create a version tag
+
+**What it does:**
+```
+1. Builds the package (wheel + sdist)
+2. Uploads to PyPI
+```
+
+**You trigger it by:**
+```bash
+git tag v2.2.0
+git push origin v2.2.0
+```
+
+---
+
+## Day-to-Day Usage
+
+### Normal Development
+
+```bash
+# Make changes
+git add .
+git commit -m "Add new feature"
+git push
+```
+
+Then check GitHub → Actions tab → See if tests pass.
+
+### If Tests Fail
+
+1. Click the failed workflow
+2. Click the failed job
+3. Read the error log
+4. Fix locally, push again
+
+### Making a Release
+
+```bash
+# 1. Update version in pyproject.toml and constants.py
+# 2. Commit the version bump
+git add .
+git commit -m "Bump version to 2.2.1"
+git push
+
+# 3. Create and push a tag
+git tag v2.2.1
+git push origin v2.2.1
+
+# 4. GitHub automatically publishes to PyPI
+```
+
+---
+
+## Reading the GitHub UI
+
+### Actions Tab
+
+```
+Repository → Actions → [List of workflow runs]
+```
+
+Each run shows:
+- ✓ Green checkmark = passed
+- ✗ Red X = failed
+- 🟡 Yellow dot = running
+
+### Pull Request Checks
+
+When you open a PR, you'll see:
+```
+┌─────────────────────────────────────────┐
+│ All checks have passed                  │
+│ ✓ test (3.10) — 45s                     │
+│ ✓ test (3.11) — 42s                     │
+│ ✓ test (3.12) — 44s                     │
+│ ✓ lint — 12s                            │
+└─────────────────────────────────────────┘
+```
+
+---
+
+## Setting Up PyPI Publishing
+
+For `release.yml` to work, you need to add a PyPI API token:
+
+### One-Time Setup
+
+1. **Create PyPI account** at https://pypi.org/account/register/
+
+2. **Generate API token:**
+   - PyPI → Account Settings → API tokens
+   - Create token (scope: entire account or just stegasoo)
+   - Copy the token (starts with `pypi-`)
+
+3. **Add to GitHub:**
+   - GitHub repo → Settings → Secrets and variables → Actions
+   - New repository secret
+   - Name: `PYPI_API_TOKEN`
+   - Value: paste the token
+
+Now `release.yml` can publish automatically.
+
+---
+
+## Common Scenarios
+
+### "Tests pass locally but fail in CI"
+
+Usually means:
+- Missing dependency in `pyproject.toml`
+- Hardcoded path that doesn't exist in CI
+- Test relies on local file
+
+### "Lint is failing"
+
+Run locally to see/fix:
+```bash
+# Check issues
+ruff check src/
+
+# Auto-fix what's possible
+ruff check --fix src/
+
+# Format code
+black src/
+```
+
+### "I want to skip CI for a commit"
+
+Add `[skip ci]` to commit message:
+```bash
+git commit -m "Update README [skip ci]"
+```
+
+---
+
+## Costs
+
+GitHub Actions is **free** for public repos.
+
+For private repos: 2,000 minutes/month free, then paid.
+
+---
+
+## Summary
+
+| Action | What Happens |
+|--------|--------------|
+| `git push` | Tests + lint run automatically |
+| Open PR | Tests must pass before merge |
+| `git tag v*` | Publishes to PyPI |
+| Check results | GitHub → Actions tab |
+
+That's it! Push code, check for green checkmarks.