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/provider.rs

@@ -174,3 +174,54 @@ impl Default for ProviderRegistry {
         Self::new()
     }
 }
+
+#[cfg(test)]
+pub mod test_utils {
+    use super::*;
+    use crate::types::{ChatRequest, ChatResponse, Message, ModelInfo, Role};
+
+    /// Mock provider for testing
+    pub struct MockProvider;
+
+    impl MockProvider {
+        pub fn new() -> Self {
+            Self
+        }
+    }
+
+    #[async_trait::async_trait]
+    impl Provider for MockProvider {
+        fn name(&self) -> &str {
+            "mock"
+        }
+
+        async fn list_models(&self) -> Result<Vec<ModelInfo>> {
+            Ok(vec![ModelInfo {
+                id: "mock-model".to_string(),
+                provider: "mock".to_string(),
+                name: "mock-model".to_string(),
+                description: None,
+                context_window: None,
+                capabilities: vec![],
+                supports_tools: false,
+            }])
+        }
+
+        async fn chat(&self, _request: ChatRequest) -> Result<ChatResponse> {
+            Ok(ChatResponse {
+                message: Message::new(Role::Assistant, "Mock response".to_string()),
+                usage: None,
+                is_streaming: false,
+                is_final: true,
+            })
+        }
+
+        async fn chat_stream(&self, _request: ChatRequest) -> Result<ChatStream> {
+            unimplemented!("MockProvider does not support streaming")
+        }
+
+        async fn health_check(&self) -> Result<()> {
+            Ok(())
+        }
+    }
+}