docs: update Learning Hub with CLI v1.0.42-1.0.43 features (#1648)

- understanding-mcp-servers.md: add Troubleshooting section with MCP
  stderr diagnostics (v1.0.42+) and /mcp show usage guidance
- copilot-configuration-basics.md: document -C startup flag (v1.0.42+),
  Auto mode server-side model routing (v1.0.43+), and /statusline
  username toggle (v1.0.43+)
- agents-and-subagents.md: add rubber-duck agent experimental feature
  documentation (v1.0.42+)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/copilot-configuration-basics.md

@@ -3,7 +3,7 @@ title: 'Copilot Configuration Basics'
 description: 'Learn how to configure GitHub Copilot at user, workspace, and repository levels to optimize your AI-assisted development experience.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-05-05
+lastUpdated: 2026-05-07
 estimatedReadingTime: '10 minutes'
 tags:
   - configuration
@@ -408,6 +408,8 @@ These files follow the same format as `config.json` and are loaded after the glo
 
 The model picker opens in a **full-screen view** with inline reasoning effort adjustment. Use the **← / →** arrow keys to change the reasoning effort level (`low`, `medium`, `high`) directly from the picker without leaving the session. The current reasoning effort level is also displayed in the model header (e.g., `claude-sonnet-4.6 (high)`) so you always know which level is active.
 
+**Auto mode and server-side model routing** (v1.0.43+): When you select **Auto** as your model, the CLI uses server-side model routing for real-time model selection. Instead of locking in a single model at session start, Auto mode evaluates each request and routes it to the most appropriate model dynamically. This means straightforward questions can be handled by a faster model while complex reasoning tasks are automatically escalated — without you needing to switch models manually.
+
 ### CLI Session Commands
 
 GitHub Copilot CLI has two commands for managing session state, with distinct behaviours:
@@ -527,12 +529,14 @@ The `/compact` command summarizes the conversation history to free up context wi
 
 > **ACP sessions (v1.0.39+)**: The `/compact`, `/context`, `/usage`, and `/env` commands are now available in ACP (Agent Coordination Protocol) sessions, allowing remote ACP clients to surface session details and manage context from within their own automated workflows.
 
-The `/statusline` command (with `/footer` as an alias) lets you control which items appear in the terminal status bar. You can show or hide individual indicators like the working directory, current branch, effort level, context window usage, and quota. The **changes** toggle shows a running count of added/removed lines for the session — useful when tracking the scope of an ongoing edit:
+The `/statusline` command (with `/footer` as an alias) lets you control which items appear in the terminal status bar. You can show or hide individual indicators like the working directory, current branch, effort level, context window usage, quota, and **active account username** (v1.0.43+). The **changes** toggle shows a running count of added/removed lines for the session — useful when tracking the scope of an ongoing edit:
 
 ```
 /statusline             # show the statusline configuration menu
 ```
 
+Toggle the **username** indicator to display which GitHub account is currently active in the footer — helpful when you work with multiple accounts or switch between personal and organization contexts.
+
 The `/keep-alive` command prevents the system from sleeping while Copilot CLI is active. This is useful during long-running agent sessions on laptops or machines with aggressive sleep settings:
 
 ```
@@ -563,6 +567,13 @@ Accepted values are `low`, `medium`, and `high`. You can also set a default via
 
 ### CLI Startup Flags
 
+The `-C <directory>` flag changes the working directory before starting, similar to `git -C` (v1.0.42+). This is useful for scripts or aliases that need to start Copilot CLI in a specific project directory without a separate `cd`:
+
+```bash
+copilot -C ~/projects/my-repo          # start in a different directory
+copilot -C ~/projects/my-repo -p "..."  # combine with prompt mode
+```
+
 The `--mode` flag (along with its aliases `--autopilot` and `--plan`) lets you launch the CLI directly in a specific agent mode without waiting for the interactive session to start:
 
 ```bash