ci(cli): add GitHub Actions workflow

[el-amin-dev]

Summary

Adds .github/workflows/cli.yml running ruff, mypy, and pytest on every push to main and every PR touching cli/**. Matrix: Python 3.10, 3.12, 3.14.

Design

Choice	Rationale
Path filter on cli/**	Monorepo scoping; backend-only PRs skip this workflow
Matrix on 3.10 / 3.12 / 3.14	Floor (supported minimum), recent stable, latest
ubuntu-latest only	Pure Python code, no OS-specific behavior
Cache pip via setup-python	Repeat installs go from about 60s to under 10s
concurrency: cancel-in-progress	Newer pushes to the same PR cancel older runs, saving runner minutes
Steps ordered ruff, mypy, pytest	Fast-fail: cheapest checks first

Verification

Locally on Python 3.14: ruff clean, mypy clean, 114 pytest passed. First CI run on this PR will verify the workflow itself.

Follow-ups

• Status badge in README (commit 2 of this PR after workflow proves green)
• Backend CI in a separate PR (out of scope here)

Closes #114