feat: add notnet crate and YubikeySource, thread Tailscale into notstrap

- New crates/notnet: TailscaleOptions, ensure_connected(), auth key chain
  (TS_AUTHKEY env → YubiKey PIV 9d → interactive prompt), install via
  apt/brew/pacman or official install.sh, peer reachability verification
- notsecrets: YubikeySource implementing IdentitySource, reads age key from
  PIV slot 9c; gated behind optional `yubikey` feature
- notstrap: Tailscale step inserted between prerequisites and clone;
  BootstrapOptions.tailscale controls override/skip; NotstrapConfig gains
  optional [tailscale] section; dotfiles_repo is now Option<String> (Tailscale
  gitea_url takes precedence); identity chain adds YubikeySource before
  Bitwarden
- Updated workspace Cargo.toml: notnet added to members and workspace deps
- Integration tests: tailscale: Some(None) to skip Tailscale in all test opts

crates/notsecrets/src/sources/mod.rs

@@ -1,8 +1,10 @@
 pub mod bitwarden;
 pub mod file;
 pub mod prompt;
+pub mod yubikey;
 
 pub use crate::ports::IdentitySource;
 pub use bitwarden::BitwardenSource;
 pub use file::FileSource;
 pub use prompt::PromptSource;
+pub use yubikey::YubikeySource;