feat(chat): add reply-to-selection with quoted context

[tyulyukov]

Summary

Implements the ability to reply to specific text selections within assistant messages. Users can:

• Select text in an agent response → a floating toolbar appears with "Reply" and "Copy" buttons
• Click "Reply" (or use Cmd/Ctrl+Shift+R) to quote the selected text as structured context
• Code block language is auto-detected when selecting from code blocks
• Quoted contexts appear as violet inline chips in the composer (with preview text and remove button)
• Sent messages with quoted context display an expandable label showing the quoted text

Key Design Decisions:

• Quoted contexts are prepended to the prompt (unlike terminal/Jira which append) as <quoted_context> XML blocks
• Transient draft state — not persisted to localStorage, cleared on thread switch or send
• 5000 character truncation limit with ...[truncated] suffix
• Deduplication by message ID and text offset to prevent duplicate quotes
• Floating toolbar uses createPortal for positioning above assistant message content

Changes

• apps/web/src/lib/quotedContext.ts (new) — Core types and utilities: QuotedContext, truncation, formatting, parsing leading quoted blocks, deduplication
• apps/web/src/components/chat/SelectionReplyToolbar.tsx (new) — Floating toolbar on text selection; detects code block context; handles Cmd/Ctrl+Shift+R and Escape
• apps/web/src/components/chat/QuotedContextInlineChip.tsx (new) — Violet inline chip in composer showing preview + remove button
• apps/web/src/components/chat/UserMessageQuotedContextLabel.tsx (new) — Expandable label in message timeline showing quoted context when user message includes it
• apps/web/src/composerDraftStore.ts — Added quotedContexts field, addQuotedContext(), removeQuotedContext(), clearQuotedContexts() actions
• apps/web/src/components/chat/MessagesTimeline.tsx — Integrated AssistantMessageContentWithReply wrapper, SelectionReplyToolbar integration, timeline label display
• apps/web/src/components/ChatView.tsx — Integrated quoted context into composer, send flow, and send state derivation
• AGENTS.md — Documented the feature architecture and key patterns

Testing

• Text selection: Select plain text in assistant message → toolbar appears above selection with Reply and Copy buttons
• Code block selection: Select text from a code block → language tag is extracted and shown in the chip preview (e.g., "Quoted code (typescript)")
• Keyboard shortcuts: Press Cmd/Ctrl+Shift+R on a selection → quoted context added to composer; press Escape → toolbar dismisses
• Copy to clipboard: Click Copy button → selected text copied without adding to composer
• Composer display: After replying to selection, a violet chip appears above editor showing preview; can remove via chip button
• Truncation: Select >5000 characters → text truncated with ...[truncated] suffix and warning logged
• Deduplication: Reply to identical selection twice → only one quote added (dedup by message ID + offset)
• Timeline display: Send message with quoted context → expandable label shows quoted text preview; can expand to see full context with language tags
• Draft lifecycle: Switch threads → quoted contexts cleared; send message → quoted contexts cleared; reload → quoted contexts NOT restored (transient)
• Toolbar positioning: Selection in different positions on screen → toolbar appears above selection, centered horizontally