Group CONTEXT.md glossary into thematic subsections

[kmcginnes]

Description

Organizes the flat ## Language glossary in CONTEXT.md into seven thematic ### subsections: Graph Data, Connections, Schema, Exploration & Views, Styling, Files & Wire Formats, and Persistence.

Pure reorganization — every term definition is byte-identical; the only changes are the added subsection headings and reordering terms into their groups. All 24 terms, the Relationships section, and the Example dialogue are unchanged.

Validation

Docs-only. Verified the set of terms and all non-heading content lines are identical to the previous version (diff empty). pnpm check:format passes.

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified `pnpm checks` passes with no errors.
• I have verified `pnpm test` passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.