Fix error message interpolation to use err.message instead of String(err)

[Copilot]

Addresses feedback on PR #41 to improve error message handling. Using String(err) produces verbose output including stack traces; err.message provides cleaner, more user-friendly error messages.

Changes

• webviewProvider.ts:1310: Changed error interpolation from String(err) to err.message in settings update handler
• agent/index.ts:70: Fixed inconsistent error handling introduced in recent refactor

// Before
vscode.window.showErrorMessage(`Failed to update setting: ${err instanceof Error ? String(err) : 'Unknown error'}`);

// After
vscode.window.showErrorMessage(`Failed to update setting: ${err instanceof Error ? err.message : 'Unknown error'}`);

Aligns with existing patterns throughout the codebase (apiService.ts, mcpServer.ts, tools/index.ts).

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