feat(terminal): align tmux + claude with code.claude.com recommendations

[chagui]

Summary

Audited the dotfiles against code.claude.com/docs/en/terminal-config. Two gaps, both addressed here:

• tmux was missing the three lines Claude Code needs to behave correctly when run inside tmux (the primary path here, since Alacritty auto-launches tmux and Ghostty is also typically used with tmux). Without them, Shift+Enter submits instead of inserting a newline, and desktop notifications + the progress bar get swallowed by tmux before reaching Ghostty.
• dot_claude/settings.json had no Notification hook. Ghostty already forwards Claude Code's desktop notification, but adding an audible cue (Glass.aiff) makes it harder to miss when the terminal is not in focus.

Ghostty's macos-option-as-alt = left and native Shift+Enter were already correct, so no changes there.

Out of scope

• Alacritty Shift+Enter binding via /terminal-setup: redundant — Alacritty here always launches tmux, so the tmux fix covers it.
• Fullscreen mode, Vim editor mode, custom themes: personal-preference toggles, not "recommendations" per the docs.

Test plan

• chezmoi diff shows only the additions in this PR
• After chezmoi apply + tmux source-file ~/.config/tmux/tmux.conf:
  • Inside tmux, claude prompt: Shift+Enter inserts a newline (does not submit)
  • When Claude finishes a task or asks for a permission prompt, Ghostty shows a desktop notification AND Glass.aiff plays
• tmux show-options -g allow-passthrough returns on
• tmux show-options -s extended-keys returns on

🤖 Generated with Claude Code