docs: add style-guide rule against "whose" for non-person antecedents - #27866

What

Adds a Word choice entry to the docs style guide, "Whose for people, not things". It restricts "whose" to people and points writers to "with", "where", or "that has" for inanimate objects and abstract concepts, since "whose" implies personhood.

Where

docs/.style/style-guide/word-choice.md, inserted right after Phrasal verbs and their noun forms, the closest existing grammar/usage rule. Formatting matches the surrounding entries: sentence-case H2, a two-sentence rationale (one sentence per line), **Do** / **Don't** blockquotes, and a documentation-only enforcement note.

Rule

"Whose" is the possessive of "who", so it implies the antecedent is a person.
When the antecedent is an inanimate object or an abstract concept, prefer "with", "where", or "that has".

Example:

• Don't: A chat whose gateway records have been pruned reports no cost.
• Do: A chat with pruned gateway records reports no cost.

Notes

• Docs-only change (Markdown under docs/, no CI or build config), so it's out of scope for /coder-agents-review; the doc-check agent covers docs-only PRs.
• The style-guide subpages are Vale-exempt, so no Vale rule ships with this. The enforcement note reads "Documentation-only. No Vale rule.", matching the other documentation-only entries.

Linear: DOCS-612

This PR was created with AI assistance (Coder Agents).