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/mcp/manager.rs

@@ -94,7 +94,10 @@ impl StdioChild {
                         if let Some(err) = resp.get("error") {
                             anyhow::bail!("MCP error: {}", err);
                         }
-                        return Ok(resp.get("result").cloned().unwrap_or(Value::Null));
+                        return Ok(resp.get("result").cloned().unwrap_or_else(|| {
+                            eprintln!("[mcp] stdio response missing 'result' field: {}", trimmed);
+                            Value::Null
+                        }));
                     }
                 }
                 Err(e) => anyhow::bail!("MCP stdio read error: {}", e),
@@ -186,7 +189,10 @@ fn call_via_http(url: &str, tool_name: &str, tool_args: &Value) -> anyhow::Resul
         .timeout(std::time::Duration::from_millis(MCP_CALL_TIMEOUT_MS))
         .connect_timeout(std::time::Duration::from_millis(MCP_CONNECT_TIMEOUT_MS))
         .build()
-        .unwrap_or_else(|_| reqwest::blocking::Client::new());
+        .unwrap_or_else(|e| {
+            eprintln!("[mcp] HTTP client builder failed: {}, using default client without timeouts", e);
+            reqwest::blocking::Client::new()
+        });
 
     let request_id: u64 = 1;
     let body = json!({
@@ -207,7 +213,10 @@ fn call_via_http(url: &str, tool_name: &str, tool_args: &Value) -> anyhow::Resul
 
     if !resp.status().is_success() {
         let status = resp.status();
-        let text = resp.text().unwrap_or_default();
+        let text = resp.text().unwrap_or_else(|e| {
+            eprintln!("[mcp] failed to read HTTP response body: {}", e);
+            String::new()
+        });
         anyhow::bail!("MCP HTTP server returned {}: {}", status, text);
     }
 
@@ -218,7 +227,10 @@ fn call_via_http(url: &str, tool_name: &str, tool_args: &Value) -> anyhow::Resul
         anyhow::bail!("MCP HTTP error: {}", err);
     }
 
-    let result = response.get("result").cloned().unwrap_or(Value::Null);
+    let result = response.get("result").cloned().unwrap_or_else(|| {
+        eprintln!("[mcp] HTTP response missing 'result' field");
+        Value::Null
+    });
     extract_text_content(&result)
 }
 
@@ -237,7 +249,10 @@ fn extract_text_content(result: &Value) -> anyhow::Result<String> {
             }
         }
     }
-    Ok(serde_json::to_string_pretty(result).unwrap_or_else(|_| result.to_string()))
+    Ok(serde_json::to_string_pretty(result).unwrap_or_else(|e| {
+        eprintln!("[mcp] failed to pretty-print result: {}", e);
+        result.to_string()
+    }))
 }
 
 #[derive(Debug, Clone)]
@@ -320,8 +335,14 @@ impl McpManager {
             tool_list.iter().filter_map(|t| {
                 Some(McpToolInfo {
                     name: t.get("name")?.as_str()?.to_string(),
-                    description: t.get("description").and_then(|v| v.as_str()).unwrap_or("").to_string(),
-                    input_schema: t.get("inputSchema").cloned().unwrap_or(serde_json::Value::Null),
+                    description: t.get("description").and_then(|v| v.as_str()).unwrap_or_else(|| {
+                        eprintln!("[mcp] tool {} missing description", t.get("name").and_then(|n| n.as_str()).unwrap_or("?"));
+                        ""
+                    }).to_string(),
+                    input_schema: t.get("inputSchema").cloned().unwrap_or_else(|| {
+                        eprintln!("[mcp] tool {} missing inputSchema", t.get("name").and_then(|n| n.as_str()).unwrap_or("?"));
+                        serde_json::Value::Null
+                    }),
                 })
             }).collect()
         } else {