feat: fold in the portable Claude Code, zsh and git config; add a drift check

Added (all now linked from their live locations):
- home_root/.claude/settings.json. The statusLine command now uses $HOME
  instead of a Linux-only absolute path.
- home_root/.claude/bin/ratecheck and home_root/.claude/skills/australis-design.
- home_config/zsh/claude-config-dir.zsh and home_root/.local/bin/claude-config-dir-init,
  the per-repo Claude Code login helper and its seeding script.
- home_root/.gitconfig and home_config/git/ignore.
- home_root/.zshenv, with the cargo env line guarded so boxes without rust start clean.

link-dotfiles --check reports drift without changing anything: tracked files whose
live copy is no longer a link (stow dry-run conflicts) and links into this repo
that dangle. Exits 1 on drift. Verified against a clean baseline and two planted
drifts, a detached .nanorc and a dangling link.

.mailmap maps every earlier identity to Vuong Hoang: the repo-local 'Your Name'
placeholder config (now removed), host-generated emails, and aider suffixes.

CLAUDE.md: the symlink warning now lists every linked file and points at --check.
README: documents what is and isn't tracked under .claude, the edit rule, and --check.

home_config/git/ignore

@@ -0,0 +1 @@
+**/.claude/settings.local.json