Fix 20_setup_tmux.sh: only run install_plugins when tpm is installed/updated

[jangroth]

Summary

• install_plugins was called unconditionally on every make install, making repeat runs slow
• Moves the call inside the DOT_FORCE/tpm-missing guard so it only runs when tpm is freshly cloned or a force-reinstall is requested

Test plan

• Run make install twice back-to-back — second run should skip plugin installation
• Run DOT_FORCE=true make install — should clone tpm and run install_plugins
• Run make install after deleting ~/.tmux/plugins/tpm — should clone tpm and run install_plugins

Closes #81

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Generated by Claude Code

[jangroth]

@claude - note this would move the command behind a check against "$DOT_FORCE". What is that being used for? Is the flag still being actively set anywhere?

[github-actions]

Claude finished @jangroth's task in 29s —— View job

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Analysis of $DOT_FORCE

• Read the script and search for $DOT_FORCE usage

$DOT_FORCE is the repo's established convention for forcing re-fetch of remote dependencies. It is actively used and set in several places:

Where it's set:

• Makefile line 51 — make reinstall runs DOT_FORCE=true ./scripts/run_all.sh
• Can also be set manually: DOT_FORCE=true ./scripts/20_setup_tmux.sh (documented in CLAUDE.md)

Where it's checked (the same guard pattern used in this PR):

• scripts/10_setup_zsh.sh:27 — gates cloning oh-my-zsh
• scripts/20_setup_tmux.sh:18 — gates cloning tpm (this PR)
• scripts/30_setup_vim.sh:15 — gates fetching pathogen.vim
• scripts/40_setup_git.sh:12 — gates fetching git-completion.bash

So moving install_plugins inside the $DOT_FORCE || ! -d tpm guard is consistent with how every other script in this repo handles remote dependencies. The flag is alive and well — make reinstall is the intended way to trigger a full re-fetch.

The change looks correct to me: install_plugins only needs to run when tpm is freshly cloned (tpm's own install script handles subsequent plugin updates when tmux starts).

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