feat: Copy Assistant Response Chain - one-click copy entire response chain including tool outputs

[ryanwyler]

Adds subtle [copy] button on completed assistant responses.

Example

▣ Build · claude-opus-4-20250514 · 17.1s [copy]

Features

• Copies entire assistant response chain including text and tool outputs
• Strips ANSI color codes from tool outputs automatically
• Copies all assistant messages in the chain (for multi-part responses)
• One-click clipboard copy with toast confirmation

Changes

• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx - Add copyAssistantResponseChain function and [copy] button to AssistantMessage