fix(install): detect shell rc files for ZDOTDIR and macOS login shells

[bezhermoso]

Summary

The installer's configure_shell_rc() hardcoded $HOME/.zshrc and $HOME/.bashrc as the only rc files to configure. This silently did nothing (no snippet appended, no warning shown) in two common scenarios:

1. ZDOTDIR is set — zsh reads $ZDOTDIR/.zshrc, not $HOME/.zshrc. This is common with dotfile managers (chezmoi, stow, yadm) and users who keep $HOME clean via XDG-style layouts.
2. macOS Bash with only .bash_profile — macOS Terminal.app and iTerm2 open login shells by default, which source ~/.bash_profile, not ~/.bashrc. Users without a .bashrc were silently skipped.

Changes

install.sh — configure_shell_rc()

• Zsh detection now respects ZDOTDIR: checks ${ZDOTDIR:-$HOME}/.zshrc
• Bash detection falls back from .bashrc to .bash_profile (first existing file wins)
• Emits a warn when no rc file is found at all, with the exact line to add manually

install.sh — Legacy migration loop

• The for rc_file in ... loop that migrates .config/wt → .wt references now checks the same expanded set of paths: ${ZDOTDIR:-$HOME}/.zshrc, $HOME/.bashrc, $HOME/.bash_profile
• When ZDOTDIR is unset, the first and second entries both resolve under $HOME; the grep guard prevents double-processing

install.sh — Post-install banner

• The "Reload your shell" instruction now dynamically shows the correct file path instead of the hardcoded source ~/.zshrc # or ~/.bashrc
• Replaces $HOME prefix with ~ for a clean display

wt.sh — _wt_ensure_sourced() error messages

• Zsh block: ~/.zshrc → ${ZDOTDIR:-~}/.zshrc
• Bash block: ~/.bashrc → ~/.bashrc (or ~/.bash_profile)

Test plan

• ./test/bats/bin/bats test/unit/ test/integration/ test/e2e/ — 167/168 pass (1 pre-existing failure in wt-remove --merged unrelated to this change)
• Manual: set ZDOTDIR to a temp dir containing a .zshrc, run installer, verify snippet lands in the correct file
• Manual: on macOS with only ~/.bash_profile (no ~/.bashrc), run installer, verify snippet lands in .bash_profile
• Manual: with neither rc file present, verify warning is printed

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