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/model/agent_def/session.rs

@@ -8,7 +8,10 @@ pub fn load_session_agents(session_dir: &Path) -> Vec<AgentDefinition> {
     }
     match std::fs::read_to_string(&agents_file) {
         Ok(content) => {
-            serde_json::from_str(&content).unwrap_or_default()
+            serde_json::from_str(&content).unwrap_or_else(|e| {
+                eprintln!("[session] failed to parse agents.json: {}", e);
+                Vec::new()
+            })
         }
         Err(_) => Vec::new(),
     }