feat: add prompt input mentions and slash commands

[Rana-Faraz]

Summary

Adds a composable, dependency-free suggestion layer to PromptInput for @ mentions and / commands while preserving its textarea and plain-text submission behavior.

Demo

@ file mentions	/ slash commands

What changed

• Adds PromptInputSuggestions, PromptInputSuggestionContent, PromptInputSuggestionItem, and PromptInputSuggestionEmpty as opt-in composition primitives.
• Exposes usePromptInputSuggestions plus trigger-boundary controls through allowedPrefixes, line position, spaces, and query length.
• Supports controlled and uncontrolled inputs, provider-backed forms, IME composition, mid-string replacement, custom insertion, and action-only items.
• Implements combobox/listbox semantics and keyboard navigation with Arrow Up/Down, Enter, Tab, and Escape.
• Documents the API with live mention and command examples, focused tests, and regenerated AI Elements skill artifacts.
• Adds a patch changeset for the new public API.

Why this approach

This addresses the suggestion discovery and plain-text insertion portion of #179 without adding a rich-editor dependency or changing the existing copy/paste and submission contract. It is complementary to the richer editor direction in #403: consumers can opt into suggestions today while keeping values as plain text.

Validation

• pnpm build --filter ai-elements --filter docs — passed; 194 static pages generated.
• pnpm test — passed; 46 files and 966 tests.
• pnpm generate-skills — passed; 48 references and 80 examples generated.
• Live docs verified in Chromium for both triggers, keyboard selection, and rendering.
• Changed TSX files pass Oxlint; git diff origin/main...HEAD --check passes.

Notes

• No breaking changes; suggestions are opt-in.
• Related to Allow PromptInput to contain rich text input with clickable @mentions and /slash commands #179.
• Complements feat(prompt-input): Add PromptInputRichtext with Lexical-powered markdown editing #403.

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