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.rs

@@ -143,3 +143,45 @@ impl McpClient for LocalMcpClient {
         self.server.call_tool(call).await
     }
 }
+
+#[cfg(test)]
+pub mod test_utils {
+    use super::*;
+
+    /// Mock MCP client for testing
+    pub struct MockMcpClient;
+
+    impl MockMcpClient {
+        pub fn new() -> Self {
+            Self
+        }
+    }
+
+    #[async_trait]
+    impl McpClient for MockMcpClient {
+        async fn list_tools(&self) -> Result<Vec<McpToolDescriptor>> {
+            Ok(vec![McpToolDescriptor {
+                name: "mock_tool".to_string(),
+                description: "A mock tool for testing".to_string(),
+                input_schema: serde_json::json!({
+                    "type": "object",
+                    "properties": {
+                        "query": {"type": "string"}
+                    }
+                }),
+                requires_network: false,
+                requires_filesystem: vec![],
+            }])
+        }
+
+        async fn call_tool(&self, call: McpToolCall) -> Result<McpToolResponse> {
+            Ok(McpToolResponse {
+                name: call.name,
+                success: true,
+                output: serde_json::json!({"result": "mock result"}),
+                metadata: HashMap::new(),
+                duration_ms: 10,
+            })
+        }
+    }
+}