refactor: split installer.go into focused step files

installer.go: 1259 → 502 lines (orchestration only)

Extracted:
- step_git.go (120 lines) — stepGitConfig, stepRestoreGit
- step_packages.go (349 lines) — preset/package selection + brew/npm install
- step_shell.go (142 lines) — stepShell, stepDotfiles, hasDotfiles
- step_system.go (186 lines) — stepMacOS, stepRestoreMacOS, stepPostInstall

No logic changes. go vet + go test + go build all pass.

Author: fullstackjam