fix(ui): describe mentions suggestions to assistive tech

[wyf027]

Summary

• associate visible Mentions suggestion hints with the textarea via aria-describedby
• preserve caller-provided aria-describedby values when internal suggestion hints are present
• add Mentions tests for accessible descriptions and input changes
• document the suggestion hint behavior with a Tailwind-based demo wrapper

Validation

• GitHub Actions CI / quality passed for head 2aab17ed3109a3296be10f67f96509b75a3c50a0.
• Passed steps: npm run lint:ui, npm run typecheck:ui, npm run test:ui, npm run build:ui, npm run build:docs.
• Test run included new src/components/form/controls/Mentions/Mentions.test.tsx coverage for suggestion descriptions, external aria-describedby, and input changes.
• Not run locally: the Codex shell executor is currently failing with Failed to create unified exec process: No such file or directory.

Notes

• codebase-memory-mcp graph tools were not exposed in this session, so this run used repository files through GitHub as the AGENTS.md fallback.
• This branch starts from current main at 11041bb72fcca72ae77d23b74ac912fc09b23bfd and avoids the still-open Carousel PR fix(ui): add carousel rotation control #82, ImagePreview PR fix(ui): improve image preview accessibility #83, and Drawer PR fix(ui): trap drawer keyboard focus #84.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.