add personal/ic separation: git identity, ssh, fly tokens, project dirs

[krmannix]

• adds `programs.git.includes` with `includeIf` to apply IC identity for all repos under `~/projects/iteratedcomputing/`
• adds `programs.ssh` with host aliases for `github.com` and `github-iteratedcomputing` (same key for now, easy to split later)
• adds `dotfiles/git/iteratedcomputing` with IC git identity (currently mirrors personal — split when needed)
• adds `dotfiles/zsh/.zshrc.d/fly.zsh` exporting `FLY_TOKEN_PERSONAL` and `FLY_TOKEN_IC` stubs, sources `~/.config/zsh/.zshrc.local` for actual values
• adds `home.activation.projectDirs` to scaffold `/projects/personal` and `/projects/iteratedcomputing` on each machine