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/yubikey.rs

@@ -0,0 +1,72 @@
+use crate::error::AgeError;
+use crate::identities::Identity;
+use crate::ports::IdentitySource;
+
+/// Reads an age private key from YubiKey PIV slot 9c (Digital Signature slot).
+///
+/// Requires the `yubikey` feature to be enabled. When the feature is absent this
+/// source is compiled out entirely — the struct still exists for type-level use but
+/// `load()` always returns an error.
+pub struct YubikeySource;
+
+impl IdentitySource for YubikeySource {
+    fn name(&self) -> &str {
+        "yubikey-piv-9c"
+    }
+
+    fn load(&self) -> Result<Box<dyn Identity>, AgeError> {
+        load_impl()
+    }
+}
+
+#[cfg(feature = "yubikey")]
+fn load_impl() -> Result<Box<dyn Identity>, AgeError> {
+    use crate::identities::x25519::X25519Identity;
+    use yubikey::{YubiKey, piv};
+
+    let mut yk = YubiKey::open().map_err(|e| AgeError::SourceError {
+        name: "yubikey-piv-9c".to_string(),
+        source: anyhow::anyhow!("cannot open YubiKey: {e}"),
+    })?;
+
+    // Slot 9c — Digital Signature slot, used here to store the age private key.
+    let data =
+        piv::read_object(&mut yk, piv::ObjectId::from(piv::SlotId::Signature)).map_err(|e| {
+            AgeError::SourceError {
+                name: "yubikey-piv-9c".to_string(),
+                source: anyhow::anyhow!("cannot read PIV slot 9c: {e}"),
+            }
+        })?;
+
+    let key = std::str::from_utf8(&data)
+        .map_err(|e| AgeError::SourceError {
+            name: "yubikey-piv-9c".to_string(),
+            source: anyhow::anyhow!("PIV slot 9c data is not valid UTF-8: {e}"),
+        })?
+        .trim()
+        .to_string();
+
+    if key.is_empty() {
+        return Err(AgeError::SourceError {
+            name: "yubikey-piv-9c".to_string(),
+            source: anyhow::anyhow!("PIV slot 9c is empty"),
+        });
+    }
+
+    let identity = X25519Identity::from_bech32(&key).map_err(|e| AgeError::SourceError {
+        name: "yubikey-piv-9c".to_string(),
+        source: anyhow::anyhow!("slot 9c content is not a valid AGE-SECRET-KEY-1: {e}"),
+    })?;
+    Ok(Box::new(identity))
+}
+
+#[cfg(not(feature = "yubikey"))]
+fn load_impl() -> Result<Box<dyn Identity>, AgeError> {
+    Err(AgeError::SourceError {
+        name: "yubikey-piv-9c".to_string(),
+        source: anyhow::anyhow!(
+            "notsecrets was compiled without the `yubikey` feature; \
+             YubikeySource is unavailable"
+        ),
+    })
+}