Define pick, chosen issue, issue that ships anyway, and linked change

[lesnik512]

The last three PRs (#29, #32, #33) added concepts that CONTEXT.md did not name, and AGENTS.md makes that file the source of the vocabulary. Docs only.

• Pick: the candidate chosen on the page as a service's release. The page, README and code used the word throughout without a definition.
• Chosen issue: a Jira issue ticked on the page, kept apart from the Issue scope that a --jira run is given.
• Issue that ships anyway: what the page marks ships anyway and lists as also ships.
• Linked change: what an issue's Web links point to, including the state and target branch a linked merge request now carries.
• Row: one more sentence, since Count Jira issues that reach a tag only through a GitLab link #32 its Jira keys also include the issues that link to it.

just lint-ci passes.