refactor: surface silent fallbacks with eprintln! logging

Add eprintln! logging before fallback values in 7 files where errors
were previously swallowed without visibility:

- [stream] malformed JSON chunk, missing usage tokens, missing tool call index
- [mcp] missing result fields, client builder failure, response body read error
- [subagent] missing API key in settings, all resolution paths exhausted
- [state] memory_dir no-parent, session_id missing, lock poisoned, store_base_dir
- [input] missing default_model for provider
- [session] corrupt agents.json parse failure
- [pkce] clock-before-epoch on random byte generation

All fallback values are preserved — this adds observability without
changing behavior for callers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/subagent/engine.rs

@@ -31,7 +31,10 @@ fn resolve_provider_config() -> (String, String, Option<String>) {
     let settings = crate::model::settings::Settings::load();
     let app_config = crate::model::app_config::AppConfig::load();
 
-    let mut api_key = settings.api_keys.get(&settings.provider).cloned().unwrap_or_default();
+    let mut api_key = settings.api_keys.get(&settings.provider).cloned().unwrap_or_else(|| {
+        eprintln!("[subagent] no API key for provider '{}' in settings, trying env/default", settings.provider);
+        String::new()
+    });
     let model = settings.model.clone();
     let base_url = app_config.providers.get(&settings.provider)
         .map(|p| p.api_base.clone());
@@ -41,7 +44,10 @@ fn resolve_provider_config() -> (String, String, Option<String>) {
             api_key = provider_cfg.api_key_env.as_ref()
                 .and_then(|env| std::env::var(env).ok())
                 .or_else(|| provider_cfg.default_api_key.clone())
-                .unwrap_or_default();
+                .unwrap_or_else(|| {
+                    eprintln!("[subagent] all API key resolution paths exhausted for '{}'", settings.provider);
+                    String::new()
+                });
         }
     }