Clean up warnings: StreamToken pub, dead oneshot code, SkipIndex

Made StreamToken pub (was pub(crate), needed by context.rs).
Removed dead API_CLIENT, get_client, sampling/priority fields
from oneshot. Suppressed pre-existing SkipIndex warning in learn.rs.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/learn.rs

@@ -22,6 +22,7 @@ const SCORE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(120);
 // ── Message building ────────────────────────────────────────────
 
 /// What to filter when building the message array for scoring.
+#[allow(dead_code)]
 enum Filter<'a> {
     None,
     SkipIndex(usize),