feat(v1.0): remove legacy MCP mode and complete Phase 10 migration

This commit completes the Phase 10 migration to MCP-only architecture by
removing all legacy mode code paths and configuration options.

**Breaking Changes:**
- Removed `McpMode` enum from configuration system
- Removed `mode` setting from `[mcp]` config section
- MCP architecture is now always enabled (no option to disable)

**Code Changes:**
- Simplified `McpSettings` struct (now a placeholder for future options)
- Updated `McpClientFactory` to remove legacy mode branching
- Always use MCP architecture with automatic fallback to local client
- Added test infrastructure: `MockProvider` and `MockMcpClient` in test_utils

**Documentation:**
- Created comprehensive v0.x → v1.0 migration guide
- Added CHANGELOG_v1.0.md with detailed technical changes
- Documented common issues (cloud model 404s, timeouts, API key setup)
- Included rollback procedures and troubleshooting steps

**Testing:**
- All 29 tests passing
- Fixed agent tests to use new mock implementations
- Updated factory test to reflect new behavior

This completes the 10-phase migration plan documented in .agents/new_phases.md,
establishing Owlen as a production-ready MCP-only TUI application.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/owlen-core/src/mcp/factory.rs

@@ -4,7 +4,7 @@
 /// Supports switching between local (in-process) and remote (STDIO) execution modes.
 use super::client::McpClient;
 use super::{remote_client::RemoteMcpClient, LocalMcpClient};
-use crate::config::{Config, McpMode};
+use crate::config::Config;
 use crate::tools::registry::ToolRegistry;
 use crate::validation::SchemaValidator;
 use crate::Result;
@@ -31,37 +31,29 @@ impl McpClientFactory {
     }
 
     /// Create an MCP client based on the current configuration
+    ///
+    /// In v1.0+, MCP architecture is always enabled. If MCP servers are configured,
+    /// uses the first server; otherwise falls back to local in-process client.
     pub fn create(&self) -> Result<Box<dyn McpClient>> {
-        match self.config.mcp.mode {
-            McpMode::Legacy => {
-                // Use local in-process client
-                Ok(Box::new(LocalMcpClient::new(
-                    self.registry.clone(),
-                    self.validator.clone(),
-                )))
-            }
-            McpMode::Enabled => {
-                // Use the first configured MCP server, if any.
-                if let Some(server_cfg) = self.config.mcp_servers.first() {
-                    match RemoteMcpClient::new_with_config(server_cfg) {
-                        Ok(client) => Ok(Box::new(client)),
-                        Err(e) => {
-                            eprintln!("Warning: Failed to start remote MCP client '{}': {}. Falling back to local mode.", server_cfg.name, e);
-                            Ok(Box::new(LocalMcpClient::new(
-                                self.registry.clone(),
-                                self.validator.clone(),
-                            )))
-                        }
-                    }
-                } else {
-                    // No servers configured – fall back to local client.
-                    eprintln!("Warning: No MCP servers defined in config. Using local client.");
+        // Use the first configured MCP server, if any.
+        if let Some(server_cfg) = self.config.mcp_servers.first() {
+            match RemoteMcpClient::new_with_config(server_cfg) {
+                Ok(client) => Ok(Box::new(client)),
+                Err(e) => {
+                    eprintln!("Warning: Failed to start remote MCP client '{}': {}. Falling back to local mode.", server_cfg.name, e);
                     Ok(Box::new(LocalMcpClient::new(
                         self.registry.clone(),
                         self.validator.clone(),
                     )))
                 }
             }
+        } else {
+            // No servers configured – fall back to local client.
+            eprintln!("Warning: No MCP servers defined in config. Using local client.");
+            Ok(Box::new(LocalMcpClient::new(
+                self.registry.clone(),
+                self.validator.clone(),
+            )))
         }
     }
 
@@ -76,9 +68,8 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_factory_creates_local_client_in_legacy_mode() {
-        let mut config = Config::default();
-        config.mcp.mode = McpMode::Legacy;
+    fn test_factory_creates_local_client_when_no_servers_configured() {
+        let config = Config::default();
 
         let ui = Arc::new(crate::ui::NoOpUiController);
         let registry = Arc::new(ToolRegistry::new(
@@ -89,7 +80,7 @@ mod tests {
 
         let factory = McpClientFactory::new(Arc::new(config), registry, validator);
 
-        // Should create without error
+        // Should create without error and fall back to local client
         let result = factory.create();
         assert!(result.is_ok());
     }