fix: resolve runtime panic in Linux keychain integration (#32)

* fix: resolve runtime panic in Linux keychain integration

- Convert Linux keychain functions from sync (rt.block_on) to async
- Remove nested runtime panic when called from async context
- Make keychain API consistent across platforms (macOS, Linux, fallback)
- Propagate async through config loading and CLI commands

Fixes panic on Linux during 'ironclaw onboard' at Step 2 (Security).

* fix: await async Config::from_env in test_heartbeat example

src/cli/tool.rs

@@ -715,7 +715,7 @@ async fn auth_tool(name: String, dir: Option<PathBuf>, user_id: String) -> anyho
     println!();
 
     // Initialize secrets store
-    let config = Config::from_env()?;
+    let config = Config::from_env().await?;
     let master_key = config.secrets.master_key().ok_or_else(|| {
         anyhow::anyhow!(
             "SECRETS_MASTER_KEY not set. Run 'ironclaw onboard' first or set it in .env"