The OpenCode reverse proxy now properly modifies response headers to
allow iframe embedding within Pulse's AI panel:
- ai_handler.go: Add ModifyResponse to strip X-Frame-Options and modify
CSP frame-ancestors from OpenCode's responses
- security.go: Skip frame-related security headers for /opencode/ paths
since the proxy manages its own headers
This fixes the "refused to connect" error when opening the AI sidebar.
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