fix(clipboard): prefer tmux load-buffer and pbcopy writes

[lavaman131]

Summary: This PR improves clipboard reliability in terminal workflows by preferring tmux-native and macOS-native copy commands while still emitting OSC 52, reducing failures in tmux sessions and avoiding AppleScript escaping edge cases on macOS.

Changes:

• Prefer tmux load-buffer -w - when running inside tmux instead of relying on tmux DCS passthrough wrapping
• Prefer pbcopy over osascript on macOS for native clipboard writes
• Simplify OSC 52 emission to direct sequence
• Extend clipboard tests to cover tmux copy behavior, pbcopy precedence, and exact OSC 52 output assertions

Technical Context:
The previous implementation wrapped OSC 52 in tmux DCS passthrough sequences, which required allow-passthrough setting. The new approach uses tmux native clipboard commands that work out of the box. On macOS, pbcopy is simpler than osascript and avoids string escaping edge cases.

[claude]

Code Review: fix(clipboard): prefer tmux load-buffer and pbcopy writes

Summary: This PR improves clipboard reliability by prioritizing native clipboard commands in tmux sessions and on macOS. The changes are well-reasoned and address real-world clipboard failures.

Strengths: Clean, well-documented implementation. Security improvement by using pbcopy with stdin instead of osascript escaping. Good test coverage for the new behaviors.

Concerns: (1) tmux -w flag requires tmux 3.2+ (April 2021) - consider fallback or documentation. (2) osascript escaping is incomplete for newlines (low priority since pbcopy is now preferred). (3) Consider edge case tests for TMUX env set without tmux command.

Verdict: LGTM with minor suggestions. The core changes are solid improvements.