feat: add claudeCode.effortLevel VS Code setting to persist default effort level

Problem

The VS Code extension has no way to set a default effort level. Every new chat session starts at "Medium" regardless of any configuration.

The CLI respects ~/.claude/settings.json ("effortLevel": "max") and the CLAUDE_CODE_EFFORT_LEVEL env var, but the VS Code extension panel ignores both. Users who always want max effort have to manually toggle it at the start of every session.

Expected behavior

A VS Code setting like claudeCode.effortLevel (values: "low", "medium", "high", "max") that sets the default for new chat sessions, similar to how claudeCode.selectedModel already works for model selection.

Current workaround

Manually clicking the effort toggle at the bottom of the chat panel every time a new session starts.

Context

• claudeCode.selectedModel already exists and works perfectly in .vscode/settings.json
• "effortLevel": "max" in ~/.claude/settings.json works for the CLI but not the extension
• CLAUDE_CODE_EFFORT_LEVEL env var works for the CLI but not the extension
• This affects users who consistently want max effort (e.g., complex codebases, thorough analysis) and shouldn't need to remember to toggle it every session