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/config.rs

@@ -242,16 +242,8 @@ impl Default for GeneralSettings {
 /// MCP (Multi-Client-Provider) settings
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct McpSettings {
-    #[serde(default)]
-    pub mode: McpMode,
-}
-
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Default)]
-#[serde(rename_all = "lowercase")]
-pub enum McpMode {
-    #[default]
-    Legacy,
-    Enabled,
+    // MCP is now always enabled in v1.0+
+    // Kept as a struct for future configuration options
 }
 
 /// Privacy controls governing network access and storage