Support persistent "max" effort level in settings.json

Summary

The VSCode extension UI exposes five effort levels (low, medium, high, xhigh, max), but the effortLevel setting in ~/.claude/settings.json accepts only four — "max" is excluded from the schema enum, so it can't be persisted as a default. Users who pick "Max" in the UI get it for the current session only; every new tab starts at xhigh.

Repro

1. Set "effortLevel": "max" in ~/.claude/settings.json.
2. Open a new Claude tab in VSCode on Opus 4.7.
3. Observe: tab opens at "Extra high" (xhigh), not "Max". The setting is silently ignored.

Evidence (extension v2.1.114)

• claude-code-settings.schema.json — effortLevel enum: ["low","medium","high","xhigh"]
• webview/index.js FN0 label map: {low:"Low", medium:"Medium", high:"High", xhigh:"Extra high", max:"Max"} — UI has "max"
• No CLAUDE_CODE_EFFORT_LEVEL env var exists to override

Requested change

Either:

• (a) add "max" to the effortLevel enum in the schema and CLI validation, or
• (b) introduce a defaultEffortLevel setting that accepts all five UI values.

Use case

Max-plan subscriber on Opus 4.7 wants every new tab to start at Max without per-tab manual selection.