feat: implemented external thinking flags and transport reasoning controls

- Added the `--external-thinking` CLI flag alongside model capability checks to gate external thinking tool availability.
- Updated Anthropic and Google transports to honor `forceReasoningOff` for native thinking-off controls.
- Renamed the `thoughts` property and parameter to `notes` across think fixtures, tools, and tests.
- Updated system prompt instructions and test suites to verify transport-specific thinking and tool activation.

packages/coding-agent/src/cli/args.ts

@@ -48,6 +48,7 @@ export interface Args {
 	serviceTier?: ServiceTierOpenAISettingValue;
 	hideThinking?: boolean;
 	advisor?: boolean;
+	externalThinking?: boolean;
 	continue?: boolean;
 	resume?: string | true;
 	fromClaude?: boolean;
@@ -255,6 +256,8 @@ export function parseArgs(inputArgs: string[], extensionFlags?: Map<string, { ty
 			result.hideThinking = true;
 		} else if (arg === "--advisor") {
 			result.advisor = true;
+		} else if (arg === "--external-thinking") {
+			result.externalThinking = true;
 		} else if (arg === "--prewalk") {
 			result.prewalk = true;
 		} else if (arg === "--no-prewalk") {