refactor: centralize config defaults and conflict checks

[Lightning00Blade]

This extracts the conflicting config argument to a separate check, to allow both defaults and conflict on the same option.

[OrKoN]

Please check the following:

• In src/config/mcp-options.ts (getMcpOptionsForViaCli and parser), apply the --viaCli default for categoryExtensions in .middleware() when !args.autoConnect && !args.browserUrl && !args.wsEndpoint (matching isolated) and update defaultDescription on isolated in getMcpOptionsForViaCli(). Keeping default: true on categoryExtensions in getMcpOptionsForViaCli() causes .check() to throw a mutual-exclusivity error whenever --viaCli (chrome-devtools start) is used with --browserUrl, --wsEndpoint, or --auto-connect, and spreading ...mcpOptions.isolated leaks defaultDescription: 'false' into CLI help.
• In src/config/browser-options.ts, src/config/mcp-options.ts (parser middleware), and src/telemetry/flagUtils.ts (computeFlagUsage), ensure isolated and channel defaults are recognized by computeFlagUsage and do not set args.channel = 'stable' when browserUrl, wsEndpoint, or executablePath is provided. Because browserOptions.isolated and browserOptions.channel only set defaultDescription (!('default' in config)), unconditionally setting args.isolated = false and args.channel = 'stable' in middleware causes computeFlagUsage to log isolated_present: true on every run and channel_present: true / channel: 'CHANNEL_STABLE' even when connecting via browserUrl/wsEndpoint or launching via executablePath.
• In src/config/mcp-options.ts (parser middleware) and src/ToolHandler.ts, resolve the legacy experimentalToonFormat fallback in ToolHandler (or before applying the 'default' default) instead of checking args.experimentalDataFormat === 'default' in middleware. Checking args.experimentalDataFormat === 'default' after Yargs applies default: 'default' causes --experimentalToonFormat to override an explicit --experimentalDataFormat=default and causes computeFlagUsage to report experimental_data_format_present: true when only --experimentalToonFormat was passed.