feat: resolve config defaults after merging CLI and config file

[Lightning00Blade]

Parse CLI arguments and the JSON config file without yargs defaults so only
explicit user intent is captured, merge them with CLI taking precedence, then
apply defaults (including the --viaCli ones). Conflict checks now only consider
explicitly provided arguments, so implicit defaults such as channel=stable can
no longer conflict with explicit flags.

Also restores --help/--version exiting via a shared buildCliParser and adds
edge-case tests for config parsing, precedence and conflicts.

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[OrKoN]

let's consider the following:

• In src/config/mcp-options.ts (stripYargsPositionalArgs and ParsedArguments), remove the (parsed as {_?: unknown}) and (parsed as {$0?: unknown}) casts (for example by using delete parsed._; delete parsed.$0; or destructuring) and omit '_' | '$0' from ParsedArguments. AGENTS.md forbids as type casting, Partial<ParsedArguments> already defines _ and $0, and keeping them on ParsedArguments misrepresents the runtime object after stripYargsPositionalArgs deletes them.
• In src/config/mcp-options.ts (parseCliArgs, parseConfigFile, and withoutDefaults), strip implies from the per-source yargs passes and validate implications (wsHeaders -> wsEndpoint, experimentalFfmpegPath -> experimentalScreencast, experimentalScreencastFps -> experimentalScreencast) on the merged explicitArgs. Evaluating implies on cliArgs and configFileArgs in isolation rejects valid split configurations (e.g., wsEndpoint in the config file with --wsHeaders on the CLI) and misses cases where the implied flag is set in the config file but negated on the CLI (--no-experimental-screencast).
• In src/config/mcp-options.ts (getCliOptions), reuse withoutDefaults(getMcpOptionsForViaCli()) before deleting the CLI-excluded options instead of duplicating the loop that populates defaultDescription and deletes default. This keeps the default-stripping logic in a single place.