[FEATURE] Add nextChat/previousChat commands for VS Code extension keybindings

Feature Request

Add claude-vscode.nextChat and claude-vscode.previousChat commands to the VS Code extension so users can cycle through conversation history via keybindings.

Motivation

Currently, the only way to switch between past conversations is through the dropdown UI in the Claude panel. There's no way to bind keyboard shortcuts for chat navigation. This makes it slow to jump between recent conversations, especially for users who prefer keyboard-driven workflows.

Proposed Solution

Expose two new commands:

• claude-vscode.nextChat — switch to the next conversation in history
• claude-vscode.previousChat — switch to the previous conversation in history

These would allow users to add keybindings like:

{
    "key": "alt+j",
    "command": "claude-vscode.previousChat",
    "when": "claude-vscode.sideBarActive"
},
{
    "key": "alt+k",
    "command": "claude-vscode.nextChat",
    "when": "claude-vscode.sideBarActive"
}

Current State

The extension exposes these commands but none for chat cycling:

• claude-vscode.newConversation
• claude-vscode.sidebar.open
• claude-vscode.editor.open
• claude-vscode.focus / claude-vscode.blur

Additional Context

Ideally these commands would also work with the activeWebviewPanelId == 'claudeVSCodePanel' when-clause for users who use Claude in an editor tab rather than the sidebar.