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/utils.rs

@@ -2,10 +2,12 @@
 
 use anyhow::Result;
 use crossterm::style::{Color, ResetColor, SetForegroundColor};
+use g3_config::Config;
 use g3_core::ui_writer::UiWriter;
 use g3_core::Agent;
 use std::path::PathBuf;
 
+use crate::cli_args::Cli;
 use crate::simple_output::SimpleOutput;
 
 /// Display context window progress bar.
@@ -89,3 +91,56 @@ pub fn setup_workspace_directory() -> Result<PathBuf> {
 
     Ok(workspace_dir)
 }
+
+/// Load configuration with CLI argument overrides applied.
+///
+/// This is the canonical function for loading config with CLI overrides.
+/// All CLI entry points should use this to ensure consistent behavior.
+pub fn load_config_with_cli_overrides(cli: &Cli) -> Result<Config> {
+    let mut config = Config::load_with_overrides(
+        cli.config.as_deref(),
+        cli.provider.clone(),
+        cli.model.clone(),
+    )?;
+
+    // Apply webdriver flag override
+    if cli.webdriver {
+        config.webdriver.enabled = true;
+    }
+
+    // Apply chrome-headless flag override
+    if cli.chrome_headless {
+        config.webdriver.enabled = true;
+        config.webdriver.browser = g3_config::WebDriverBrowser::ChromeHeadless;
+
+        // Run Chrome diagnostics on first use
+        let report =
+            g3_computer_control::run_chrome_diagnostics(config.webdriver.chrome_binary.as_deref());
+        println!("{}", report.format_report());
+    }
+
+    // Apply safari flag override
+    if cli.safari {
+        config.webdriver.enabled = true;
+        config.webdriver.browser = g3_config::WebDriverBrowser::Safari;
+    }
+
+    // Apply no-auto-compact flag override
+    if cli.manual_compact {
+        config.agent.auto_compact = false;
+    }
+
+    // Validate provider if specified
+    if let Some(ref provider) = cli.provider {
+        let valid_providers = ["anthropic", "databricks", "embedded", "openai"];
+        if !valid_providers.contains(&provider.as_str()) {
+            return Err(anyhow::anyhow!(
+                "Invalid provider '{}'. Valid options: {:?}",
+                provider,
+                valid_providers
+            ));
+        }
+    }
+
+    Ok(config)
+}