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/agent.rs

@@ -364,12 +364,14 @@ impl AgentExecutor {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::mcp::test_utils::MockMcpClient;
+    use crate::provider::test_utils::MockProvider;
 
     #[test]
     fn test_parse_tool_call() {
         let executor = AgentExecutor {
-            llm_client: Arc::new(crate::provider::MockProvider::new()),
-            tool_client: Arc::new(crate::mcp::MockMcpClient::new()),
+            llm_client: Arc::new(MockProvider::new()),
+            tool_client: Arc::new(MockMcpClient::new()),
             config: AgentConfig::default(),
         };
 
@@ -397,8 +399,8 @@ ACTION_INPUT: {"query": "Rust programming language"}
     #[test]
     fn test_parse_final_answer() {
         let executor = AgentExecutor {
-            llm_client: Arc::new(crate::provider::MockProvider::new()),
-            tool_client: Arc::new(crate::mcp::MockMcpClient::new()),
+            llm_client: Arc::new(MockProvider::new()),
+            tool_client: Arc::new(MockMcpClient::new()),
             config: AgentConfig::default(),
         };