agent: finish tracing migration for hotkey logs

Follow-up to 184214b. The eprintln→tracing sweep surfaced 6 existing
log::* calls in the hotkey crate that were silent at runtime — the
workspace builds tracing-subscriber without the tracing-log feature, so
log:: events never reached the tracing pipeline.

Migrating direct rather than adding a LogTracer bridge: the repo is
already standardising on tracing, the bridge adds a second logger init
path with "already initialised" edge cases, and the bridge would
indiscriminately route all log records (including future third-party
chatter), not just these known sites.

Migrated (6 sites, 2 files):
- crates/hotkey/src/linux.rs (5) — read /dev/input error, device open
  debug, device-attached info, device-listener-ended warn, event-channel-
  closed warn
- crates/hotkey/src/stub.rs (1) — non-Linux no-op info

Also removed the now-unused log = "0.4" dependency from
crates/hotkey/Cargo.toml. magnotia_hotkey=info filter target was
already added to init_tracing in 184214b, so these events emit at the
default level immediately.

Verification:
- cargo fmt --all -- --check
- cargo check -p magnotia-hotkey — clean
- cargo check -p magnotia — clean
- rg 'log::|eprintln!' crates/hotkey/src/ src-tauri/src/commands/ crates/ai-formatting/src/ — zero hits

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/hotkey/src/stub.rs

@@ -19,7 +19,7 @@ pub struct EvdevHotkeyListener;
 
 impl EvdevHotkeyListener {
     pub fn start(_combo: HotkeyCombo, _event_tx: mpsc::Sender<HotkeyEvent>) -> Self {
-        log::info!("evdev hotkey listener is a no-op on this platform");
+        tracing::info!("evdev hotkey listener is a no-op on this platform");
         Self
     }