index --watch should propagate sub-run failures instead of discarding exit codes

[Widthdom]

Problem

IndexWatchRunner.InvokeSubRunAndEmit calls IndexCommandRunner.Run(args.ToArray(), jsonOptions) while stdout is redirected, but the returned exit code is not stored or checked. RunCore can continue emitting normal watch summaries even when a full or partial rescan failed, which makes watch-mode failures easy to miss in automation.

Suggested fix

Capture the sub-run exit code, include it in watch events and human summaries, and make watch mode mark the batch as failed or degraded when the sub-run fails.

Evidence

• src/CodeIndex/Cli/IndexWatchRunner.cs: InvokeSubRunAndEmit
• IndexCommandRunner.Run(args.ToArray(), jsonOptions)
• no stored exitCode in the watch sub-run path

[Widthdom]

Work started on branch fix-issue2903-2902.