ci: add CI pipeline with linting, tests, secrets scan, and Dependabot

[LewallenAE]

Adds automated CI pipeline as mentioned in the README ("CI/CD pipelines are not yet included. Contributions welcome.")

What's included

• Lint & Format — Ruff for linting and format checking
• Type checking — mypy (non-blocking for now)
• Secrets scan — Gitleaks to catch accidentally committed API keys
• Tests — pytest matrix across Python 3.10, 3.11, 3.12 with coverage
• Docker build — Validates Dockerfile builds successfully
• Dependabot — Weekly dependency updates for pip and GitHub Actions

Files added

• .github/workflows/ci.yml
• .github/dependabot.yml
• ruff.toml

[Copilot]

Pull request overview

This PR adds a comprehensive CI/CD pipeline to the NLWeb project, addressing the gap mentioned in the README. The implementation includes automated linting, testing, security scanning, and dependency management through GitHub Actions.

Changes:

• Added CI workflow with linting (Ruff), type checking (mypy), testing across Python 3.10-3.12, secrets scanning (Gitleaks), and Docker build validation
• Configured Dependabot for automated weekly dependency updates for Python packages and GitHub Actions
• Added Ruff configuration file for consistent code formatting and linting rules

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 12 comments.

File	Description
.github/workflows/ci.yml	Defines the complete CI pipeline with jobs for linting, security scanning, matrix testing, and Docker build validation
.github/dependabot.yml	Configures automated dependency updates for pip and GitHub Actions with weekly schedules and grouped updates
ruff.toml	Establishes linting and formatting rules for the Python codebase with conservative initial configuration

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[LewallenAE]

@chelseacarter29 Ready for review! Fixed the ruff.toml configuration error and replaced gitleaks with TruffleHog (free alternative) to resolve the CI failures.