fix(installer): preserve existing oh-my-zsh installation

[eezzekl]

Closes #187

Summary

• The installer copied the vendored GentlemanZsh/.oh-my-zsh snapshot over ~/.oh-my-zsh unconditionally, which dirties the tracked files of a real Oh My Zsh clone and makes omz update fail on the autostash pop.
• Oh My Zsh is now installed through its official installer only when ~/.oh-my-zsh is missing, and an existing installation is never written into.
• The vendored copy is removed, since it carried no project-owned code and Oh My Zsh manages its own checkout.

Changes

File	Change
installer/internal/system/exec.go	New PathExists helper next to EnsureDir. Resolves symlinks on purpose, so a symlinked ~/.oh-my-zsh counts as installed.
installer/internal/tui/installer.go	New shouldInstallOhMyZsh; the system.CopyDir call in case "zsh" is replaced by the guard plus the official installer.
installer/internal/tui/installation_steps_test.go	TestShouldInstallOhMyZsh covering missing, real-directory and symlinked-directory states.
docs/manual-installation.md	cp -rf GentlemanZsh/.oh-my-zsh ~/ replaced with a guarded official-installer invocation.
GentlemanZsh/.oh-my-zsh	Removed — 1021 files, ~11 MB.

The installer call is:

ZSH=<home>/.oh-my-zsh RUNZSH=no CHSH=no KEEP_ZSHRC=yes \
  sh -c "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)"

KEEP_ZSHRC=yes is what preserves the .zshrc copied a few lines above; RUNZSH/CHSH keep it non-interactive. All three are documented in Oh My Zsh's tools/install.sh. This introduces no new dependency — the installer already clones tpm (installer.go:935), Alacritty, and runs the Homebrew installer via curl.

Why the vendored copy is safe to remove

Every bundled file was hashed and checked against Oh My Zsh's own history:

h=$(git hash-object "GentlemanZsh/.oh-my-zsh/$f")
git -C ~/.oh-my-zsh cat-file -e "$h"   # exists => unmodified upstream file

1015 of 1019 files matched an identical upstream blob. The remaining 4 are plugins/fig/ and plugins/rbfu/, also upstream files, whose blobs are missing only because upstream deleted those plugins. rg -i gentleman over the tree returns nothing, and the directory has 3 commits in the entire repository history, all imports. There is nothing to migrate into $ZSH_CUSTOM.

Worth noting for anyone who resolved these conflicts by hand: the bundle_install() function in the bundled plugins/bundler/bundler.plugin.zsh is deprecated Oh My Zsh code, not a Gentleman.Dots function. Upstream replaced it with alias bi="bundle install".

Test plan

• go test ./... -skip Golden -count=1 — system 5.7s, tui 71.8s, trainer OK
• go build ./cmd/gentleman-installer and --help / --version smoke test
• go vet ./... clean
• New unit test covers both branches of the guard without requiring network access
• No .golden snapshot references Oh My Zsh, so the macOS job is unaffected
• Every E2E Dockerfile already installs curl and git

Note on E2E

e2e_test.sh runs --shell=zsh three times and its cleanup never removes $HOME/.oh-my-zsh, so the first run exercises the install path and the later runs exercise the guard — both branches get covered.

One deliberate trade-off: the zsh E2E now needs to reach GitHub to install Oh My Zsh, where it previously only copied files. I did not soften the failure into a warning, because .zshrc runs source $ZSH/oh-my-zsh.sh — silently skipping the install would leave the user with a shell that breaks at startup. Failing the step is the safer behavior, but it is a new network dependency in that job and worth knowing about.