@rail44.dev/goose

In `docs/design.md` ONLY, fix one stale file path. The file `src/mention.rs` moved to `quacker-core/src/mention.rs` in the quacker-core extraction (PR #312); `src/mention.rs` no longer exists at the top level.

At line ~1588 — "mention is parsed server-side (`src/mention.rs`) and stored in the …" — replace the path `src/mention.rs` with `quacker-core/src/mention.rs`.

Constraints:
- Edit ONLY `docs/design.md`, and change ONLY that one `src/mention.rs` → `quacker-core/src/mention.rs` path string. Keep all surrounding prose and every other line byte-for-byte identical.
- Do NOT touch any other file or any other path.

This is a mechanical path update for a moved file.