refactor(g3-cli): eliminate code-path aliasing in config and project content loading

Consolidate duplicated logic into canonical shared functions:

- Extract load_config_with_cli_overrides() to utils.rs
  - Was duplicated in lib.rs and accumulative.rs with subtle differences
  - lib.rs version had Chrome diagnostics + provider validation
  - accumulative.rs version was missing both
  - Now all callers use the complete canonical implementation

- Extract combine_project_content() to project_files.rs
  - Was duplicated inline in lib.rs and agent_mode.rs
  - Simplified implementation using iterator flatten
  - Added unit tests for all cases

This eliminates drift risk where the duplicated implementations
could diverge over time (accumulative.rs was already missing
Chrome diagnostics and provider validation).

Agent: fowler

crates/g3-cli/src/agent_mode.rs

@@ -7,7 +7,7 @@ use tracing::debug;
 use g3_core::ui_writer::UiWriter;
 use g3_core::Agent;
 
-use crate::project_files::{read_agents_config, read_project_memory, read_project_readme};
+use crate::project_files::{combine_project_content, read_agents_config, read_project_memory, read_project_readme};
 use crate::simple_output::SimpleOutput;
 use crate::ui_writer_impl::ConsoleUiWriter;
 
@@ -177,23 +177,11 @@ pub async fn run_agent_mode(
     ));
 
     // Combine all content for the agent's context
-    let combined_content = {
-        let mut parts = Vec::new();
-        if let Some(agents) = agents_content_opt {
-            parts.push(agents);
-        }
-        if let Some(readme) = readme_content_opt {
-            parts.push(readme);
-        }
-        if let Some(memory) = memory_content_opt {
-            parts.push(memory);
-        }
-        if parts.is_empty() {
-            None
-        } else {
-            Some(parts.join("\n\n"))
-        }
-    };
+    let combined_content = combine_project_content(
+        agents_content_opt,
+        readme_content_opt,
+        memory_content_opt,
+    );
 
     // Create agent with custom system prompt
     let ui_writer = ConsoleUiWriter::new();