fix(overlay): pass env vars through temp file for tmux/kitty/wezterm popups

[nnemirovsky]

Summary

• tmux display-popup / kitty @launch / wezterm cli split-pane start a fresh shell that does NOT inherit exported env vars from the caller
• The overlay dialog was silently failing because it couldn't see PASSTHRU_OVERLAY_RESULT_FILE, PASSTHRU_OVERLAY_TOOL_NAME, etc.
• Fix: write env vars to a temp file using printf '%q' (shell-safe quoting), source it inside the popup, then delete it
• Also fixes CI: replaced broken-symlink PATH masking with sanitized PATH (symlinks only needed utilities, excludes tmux/kitty/wezterm)

Test plan

• bats tests/*.bats passes (714 tests, 0 failures)
• Manual tmux popup test with TEST_ANSWER=yes_once produces result file
• Live overlay popup with real keypress