feat: shell + dotfiles setup for a fresh box

Two layers with a deliberate privilege split: setup.sh does the root work
(apt, rustup, cargo tools, starship, chsh) and hands off to chezmoi, which
owns everything under $HOME.

Tools are built with `cargo install --locked` to match the versions currently
running rather than whatever apt ships, at the cost of a 10-20 min cold setup.

Excluded on purpose: the atuin sync key and ~/.git-credentials (secrets),
~/.claude/settings.json (per-box decision), fnm/opencode conf.d (out of scope),
and fish_variables (machine-local).

chezmoi tracks only the executable and private attributes and derives modes
from the umask, so setup.sh pins umask 022 and atuin uses private_ to keep
0700/0600. Without both, a 002 umask silently relaxes them to 0775/0664.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

home/.chezmoiignore

@@ -0,0 +1,27 @@
+# Never manage these — they are secrets or machine-local runtime state.
+# chezmoi will neither read them into source state nor write them to the target.
+
+# Git credential store (plaintext tokens — .gitconfig sets helper=store)
+.git-credentials
+
+# Atuin runtime state + sync encryption key.
+# The key is tracked by hand in secrets.yml; restore with `atuin login`.
+.local/share/atuin/**
+
+# Claude runtime state — history, per-project memory, session data.
+# Only statusline-command.sh is managed. settings.json is deliberately NOT
+# tracked: it pins the model and sets skipDangerousModePermissionPrompt, which
+# is a per-box decision. Set it by hand on each machine.
+.claude/projects/**
+.claude/todos/**
+.claude/statsig/**
+.claude/.caveman-active
+.claude/settings.json
+.claude/settings.local.json
+
+# Fish machine-local state (generated by fish itself)
+.config/fish/fish_variables
+
+# Out of scope for this repo (installed by their own installers, if at all)
+.config/fish/conf.d/fnm.fish
+.config/fish/conf.d/dtop.env.fish