fix(clipboard): add OSC 52 fallback for SSH/tmux environments

[wonderflow]

Problem

Clipboard copy operations (e.g., Ctrl+X Y to copy assistant message) silently fail when running opencode over SSH or in tmux on a remote Linux server without X11/Wayland display.

Root cause: The fallback path uses clipboardy, which relies on xsel/xclip and requires a local display ($DISPLAY or $WAYLAND_DISPLAY). In headless SSH/tmux environments, these tools aren't available or don't work, causing clipboard operations to fail silently.

Solution

Add OSC 52 escape sequence support as a fallback when clipboardy fails.
OSC 52 is a terminal escape sequence standard that allows applications to set the system clipboard through the terminal emulator. It's supported by:

• iTerm2
• kitty
• Alacritty
• Windows Terminal
• tmux (with set-clipboard option)
• Many other modern terminal emulators

Changes

• Added copyViaOSC52() function that sends OSC 52 escape sequences
• Handles tmux passthrough wrapping when $TMUX is set
• Modified fallback logic: try clipboardy first → fall back to OSC 52 on failure

Behavior

Environment	Before	After
macOS local	osascript ✅	osascript ✅ (unchanged)
Linux + X11/Wayland	xclip/xsel/wl-copy ✅	xclip/xsel/wl-copy ✅ (unchanged)
Windows	powershell ✅	powershell ✅ (unchanged)
SSH/tmux (no display)	clipboardy ❌ (silent fail)	clipboardy → OSC 52 ✅

No breaking changes - existing behavior preserved for all native clipboard methods.

Testing

1. SSH into a remote Linux server
2. Run opencode in tmux with iTerm2 integration
3. Get a response from the assistant
4. Press Ctrl+X Y to copy the assistant's message
5. Cmd+V on local machine to verify clipboard content

Prerequisite

• Enable "Applications in terminal may access clipboard" in iTerm2 Preferences → General → Selection

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Results

No duplicates found.

The search for the current PR (7028: "fix(clipboard): add OSC 52 fallback for SSH/tmux environments") returned only the PR itself.

While there are other clipboard-related PRs in the repository:

• PR fix: validate DIBv5 format before fallback to DIB in Windows clipboard #2861 - Windows clipboard format validation
• PR feat: add native X11 clipboard support for Linux #5908 - X11 clipboard support for Linux
• PR feat: auto-compress clipboard images to avoid API size limits #6455 - Clipboard image compression

These are addressing different clipboard issues and platforms, not the same SSH/tmux OSC 52 fallback feature being implemented in PR #7028.