## Summary
Adds AI response copy functionality to chat messages that preserves
formatting and converts Dyad-specific tags to clean, readable markdown.

## Changes
- **New `useCopyToClipboard` hook**: Parses Dyad tags and converts them
to professional markdown format
- **Updated `ChatMessage` component**: Positions copy button on left
side of approval status
- **Dyad tag conversion**: Transforms custom tags to readable format:
  - `<dyad-write>` → `### File: path/to/file.js` + code block
  - `<dyad-edit>` → `### Edit: path/to/file.js` + code block  
  - `<dyad-execute-sql>` → `### Execute SQL` + ```sql block
  - `<think>` → `### Thinking` + content

## Features
- ✅ Automatic programming language detection from file extensions  
- ✅ Professional markdown formatting with proper headings and code
blocks
- ✅ Tooltip showing "Copied" confirmation
- ✅ Reuses existing DyadMarkdownParser logic for consistency

closes (#1290)
    
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## Summary by cubic
Adds a Copy button to assistant messages that copies a clean Markdown
version of the response by converting Dyad tags and preserving code
blocks. Improves shareability and removes Dyad-only markup; addresses
Linear #1290.

- **New Features**
- Added useCopyToClipboard hook that parses Dyad tags to Markdown,
auto-detects code language, and cleans spacing.
- Updated ChatMessage to show a Copy button (with Copy/Copied tooltip)
to the left of approval status; disabled while streaming.
- Tag conversions: think → "### Thinking"; dyad-write/edit → "###
File/Edit: path" + fenced code; dyad-execute-sql → "### Execute SQL" +
sql block; other Dyad tags map to concise headings; chat-summary/command
are omitted.
- Added e2e tests for clipboard copy, Dyad tag stripping/formatting, and
tooltip states.

<!-- End of auto-generated description by cubic. -->