[Backport #2016 to 5.x] Add domain glossary (CONTEXT.md)

[github-actions]

Automated backport of #2016 to branch 5.x.

Do not modify this pull request.

Summary

• Adds CONTEXT.md at the repo root — a domain glossary defining core project terminology (Annotation, Analysis, Context, Generator, Processor, etc.) with avoid-terms, relationships, and resolved ambiguities
• Removes outdated TokenAnalyser references from docs/guide/under-the-hood.md
• Aligns docs terminology: drops "enrich" in favour of "augment"

Test plan

• Review glossary terms for accuracy
• Verify no broken links in docs

🤖 Generated with Claude Code

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-2016-to-5.x
git worktree add --checkout .worktree/backport-2016-to-5.x backport-2016-to-5.x
cd .worktree/backport-2016-to-5.x
git reset --hard HEAD^
git cherry-pick -x d9b584df73670bf390e1dd8ca8b74c44847de00b