feat(session): headless --pair, so a box with only SSH can enrol

`punktfunk-session --pair <PIN> --connect host[:port]` runs the SPAKE2
ceremony with no window and no toolkit, prints the same
`paired <addr>:<port> fp=<hex>` line as `punktfunk-client --pair`, and
exits. Until now the PIN ceremony lived only in the GTK shell or the Skia
console, so enrolling an embedded/kiosk client meant installing a desktop
on it — or copying the identity store by hand.

Dispatches above every graphics call (the machine may have no display) and
is in the `--no-default-features` build: enrolling must never be the reason
a minimal image pulls in Skia. `--name` defaults to the hostname instead of
the desktop path's hardcoded "Steam Deck".

`forget_placeholder` and `device_name` move into pf_client_core::trust so
the two binaries share one implementation rather than the session
re-deriving them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

clients/linux/src/cli.rs

@@ -3,7 +3,7 @@
 //! scenes.
 
 use crate::app::AppModel;
-use crate::trust::{KnownHost, KnownHosts};
+use crate::trust::{forget_placeholder, KnownHost, KnownHosts};
 use crate::ui_hosts::{ConnectRequest, HostsMsg};
 use gtk::glib;
 use gtk::prelude::*;
@@ -252,18 +252,6 @@ fn probe_all(hosts: &[KnownHost]) -> Vec<bool> {
     )
 }
 
-/// Drop an fp-less placeholder for `addr:port` (see `headless_pair`). No-op when none exists.
-fn forget_placeholder(addr: &str, port: u16) {
-    let mut known = KnownHosts::load();
-    let before = known.hosts.len();
-    known
-        .hosts
-        .retain(|h| !(h.fp_hex.is_empty() && h.addr == addr && h.port == port));
-    if known.hosts.len() != before {
-        let _ = known.save();
-    }
-}
-
 /// `--list-hosts [--probe]` — the saved known-hosts store as JSON (the store the Decky plugin
 /// renders). With `--probe`, each host carries an `online` bool from a live reachability probe
 /// (mDNS-independent); without it, `online` is `null` (unknown — the caller falls back to its