[zaccesss]

Closes #27

New modules

Each one follows the same exercise, extra practice and self-test split as modules 01 to 10.

Module	What it covers
11 GitLab	Merge requests, a .gitlab-ci.yml pipeline (example file included), protected branches and glab
12 Signing and security	SSH commit signing, the Verified badge, signed tags, ignoring secrets and push protection
13 Undoing mistakes	A table of which undo fits which mistake, the three resets, revert, reverting a merge and recovering with the reflog
14 GitHub Projects	Boards, custom fields, built-in workflows, sub-issues, milestones, iterations and gh project
15 Newer Git and gh	gh issue develop, --parent, pr checks --watch, pr update-branch, pr revert and ruleset check, plus git config get, repo info, last-modified, range-diff, maintenance, backfill and Actions concurrency

Every command in module 15 was checked against Git 2.54 and gh 2.102.

Existing content

• actions/checkout from v4.4.0 to v7.0.1, markdownlint-cli2-action pinned to a commit at v24.2.0
• The rulesets row in docs/github-ui-vs-terminal.md corrected: rulesets can be created through the REST API
• CONTRIBUTING.md moved to git switch with an example that fits this repository
• Glossary, command reference, module index, README progress table and changelog updated to match

[zaccesss]

Checked locally: markdownlint clean across 73 files, every relative link resolves, all three YAML files parse and each module 15 command exists in the installed Git and gh.