Tracing: carry span into insight panic-isolation tasks; root the watcher boot probe (#143)

Found by the parity capture: both insight-generation handlers run the
generation inside a nested tokio::task::spawn (panic isolation) that was
not instrumented, so the background ai.insight.* trace still split off
from its 202 request. The watcher's boot-time availability probe ran
before the first tick span, leaving its count_for_library DB spans as
roots; it now opens `watcher.initial_probe`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/watcher.rs

@@ -394,6 +394,7 @@ pub fn watch_files(
         // Disabled libraries skip the probe entirely — they should
         // never enter the health map (treated as out-of-scope).
         {
+            let _probe = tracing::info_span!("watcher.initial_probe").entered();
             let libs = libs_lock.read().unwrap_or_else(|e| e.into_inner());
             for lib in libs.iter() {
                 if !lib.enabled {