feat(vscode): add setting to toggle token/cost counter visibility

[awhite0030]

Description

Brief description of what this PR does

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Docs-only or internal chores need no changeset (or run pnpm changeset --empty to note that intentionally).

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)

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Root cause: The token and cost counter is shown unconditionally, which creates visual noise for some users.
Fix: Added a VS Code setting nanocoder.showTokenUsage (defaulting to false) and plumbed it into the webview payload so that the UI only renders the counter when enabled. Created a changeset.
Validation: Ran pnpm run build, pnpm test:format, pnpm test:lint, pnpm test:types, pnpm test:knip, and targeted tests pnpm test:ava plugins/vscode/src/. All passed. Validated changeset logic with node scripts/validate-changesets.js.
Fixes #1096

Fixes #1096