docs: add GitLab CI integration guide

[Silvren]

Summary

• add a copy-ready GitLab CI job that installs and runs AgentGuard
• document severity thresholds, exit codes, artifact retention, and GitLab integration limitations
• validate the checked-in YAML example in the test suite

Tests

• ruff format --check src tests
• ruff check src tests
• mypy src
• pytest (47 passed, 90.98% coverage)

Fixes #10

[amic25]

Taking this one — thanks. The tests/test_gitlab_ci_docs.py check is the part I appreciate most: a documented pipeline that nothing verifies goes stale silently, and this keeps the YAML honest.

Two notes before merge, neither blocking:

1. Exit codes changed after Make the scanner safe to run on untrusted code, and its findings measurable #17. There are now three: 0 clean, 1 findings at or above --fail-on, 2 the scan did not complete. 2 outranks 1 deliberately, so CI can tell "found problems" from "the tool broke". If the GitLab job treats any non-zero as a finding, it will report a crashed scan as a security result. Worth a line in the guide.
2. There's now a --fail-on-incomplete flag for pipelines that need to fail when a line was too long to be read in full. Optional, but GitLab users scanning bundled JS may want it.

Note #15 covers overlapping ground — I've replied there suggesting the two be combined rather than one dropped.