Add tailscale-exit-node build type across all three platforms

New build type dedicated to Tailscale exit-node capability, wired up for
linode/proxmox/lxc like every other build type (the lxc variant is the one
actually intended for deployment). Kept separate from the "server" host
rather than bundling exit-node capability onto it.

Trimmed modules/tailscale/exit-node.nix down to pure exit-node behavior:
dropped the old --advertise-routes=${vars.lanCidr} bundling (meaningless
for a Linode-hosted VPS with no path to the LAN), and switched
extraUpFlags -> extraSetFlags. Confirmed against nixpkgs' tailscale.nix
that extraUpFlags is only applied by tailscaled-autoconnect, which itself
only runs when services.tailscale.authKeyFile is set -- nothing in this
repo sets one, so the old flags would never have actually been applied.
extraSetFlags runs unconditionally via tailscaled-set on every boot, so
--advertise-exit-node self-reapplies once the operator has done the
one-time manual `tailscale up` auth.

Verified: all three new targets eval cleanly, nixpkgs-fmt/statix clean,
and a dry-run build of lxc-tailscale-exit-node's tarball resolves its full
closure including tailscaled-set.service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

CLAUDE.md

@@ -133,9 +133,10 @@ nixosSystem {
 ```
 
 Platforms: `linode`, `proxmox`, `lxc`. Build types: `minimal`, `nix-cache`,
-`server`, `docker`, `gui`, `pxe-boot`. Not every combination is built — e.g.
-`pxe-boot` has no `linode` variant (PXE/DHCP/TFTP need LAN L2 adjacency a
-Linode VPS doesn't have). Treat `flake.nix`'s `generatedTargets` as the source
+`server`, `docker`, `gui`, `pxe-boot`, `tailscale-exit-node`. Not every
+combination is built — e.g. `pxe-boot` has no `linode` variant (PXE/DHCP/TFTP
+need LAN L2 adjacency a Linode VPS doesn't have). Treat `flake.nix`'s
+`generatedTargets` as the source
 of truth for which hosts exist — `README.md`, `AGENTS.md`,
 `docs/flake-lock-automation.md`, and the CI eval workflows
 (`.github/workflows/check-nixos.yml`, `.gitea/workflows/check-nixos.yml`) list