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/Cargo.toml

@@ -23,6 +23,11 @@ hmac             = "0.12"
 rand             = "0.8"
 zeroize          = "1"
 curve25519-dalek = "4"
+yubikey          = { version = "0.8", optional = true }
+
+[features]
+default = []
+yubikey = ["dep:yubikey"]
 
 [dev-dependencies]
 tempfile = { workspace = true }