perf: speed up startup from ~15s to ~2s (#280)

Three high-impact changes eliminate most startup latency:

1. Enable wasmtime persistent compilation cache — call
   cache_config_load_default() so compiled native code is serialized to
   disk (~/.cache/wasmtime). Subsequent startups deserialize instead of
   recompiling, dropping the WASM phase from ~13s to <1s.

2. Cache compiled Component in PreparedModule — store the compiled
   wasmtime::component::Component directly instead of raw bytes.
   Eliminates ~2.6s recompilation on every first tool/channel execution.

3. Move blocking housekeeping to background tasks — embedding backfill
   (~1.3s of failing HTTP calls) and stale job cleanup are fire-and-forget
   work that no longer blocks the critical startup path.

Also: deduplicate Workspace creation in main.rs (two identical instances
reduced to one), and replace leftover println! in session validation with
tracing calls.

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/llm/session.rs

@@ -157,14 +157,14 @@ impl SessionManager {
         }
 
         // Token exists, validate it by calling /v1/users/me
-        println!("Validating session...");
+        tracing::debug!("Validating session...");
         match self.validate_token().await {
             Ok(()) => {
-                println!("Session valid.");
+                tracing::debug!("Session valid");
                 Ok(())
             }
             Err(e) => {
-                println!("Session expired or invalid: {}", e);
+                tracing::info!("Session expired or invalid: {}", e);
                 self.initiate_login().await
             }
         }