feat(chat): add copy button to agent responses

[vybe]

Context

Client feedback (Paradigm Life, April 2026) identified missing copy functionality on chat responses.

Current State

ChatBubble.vue renders assistant messages but has no copy-to-clipboard button.

Requested

Add a copy button to every assistant response bubble that copies the raw markdown content to clipboard.

Acceptance Criteria

• Copy button appears on hover/focus for assistant messages
• Clicking copies raw content to clipboard
• Visual feedback on successful copy (checkmark or toast)
• Works in both AgentDetail and PublicChat views

Validation Required

• Confirm this matches the standard pattern users expect from Claude.ai and similar tools

References

• Client feedback: "No one-click copy on agent responses"
• Effort: Low
• Priority: Quick win

/label enhancement,ui,P1b