#13638: fix: pass delivery context to cron isolated agent subagents

## Problem Subagents spawned from cron isolated sessions lose the Slack delivery context, causing `Delivering to Slack requires target` errors when the announce flow tries to deliver results. ## Root Cause In `src/cron/isolated-agent/run.ts`, the call to `runEmbeddedPiAgent()` was missing `messageTo` and `messageThreadId` parameters. These values were correctly resolved from `resolvedDelivery` but never passed through, so all downstream tools (including `sessions_spawn`) received `undefined` as the delivery target. The cron job's own direct announce path correctly uses `resolvedDelivery.to`. The bug is specifically in the subagent spawn path where delivery context is lost at the `runEmbeddedPiAgent` boundary. ## Fix Pass `messageTo` and `messageThreadId` from `resolvedDelivery` to `runEmbeddedPiAgent()`, matching the pattern used in `src/auto-reply/reply/agent-runner-execution.ts`. 2 lines added. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes a delivery context propagation bug in cron isolated agent sessions. The fix adds `messageTo` and `messageThreadId` parameters to the `runEmbeddedPiAgent()` call, ensuring that subagents spawned from cron jobs inherit the Slack delivery context from their parent. - Adds `messageTo: resolvedDelivery.to` and `messageThreadId: resolvedDelivery.threadId` to the `runEmbeddedPiAgent()` call in `src/cron/isolated-agent/run.ts:410-411` - Matches the pattern used in `src/auto-reply/reply/agent-runner-execution.ts:264-265` - Prevents `Delivering to Slack requires target` errors when subagents attempt to announce results <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The fix is a straightforward 2-line addition that passes existing delivery context parameters to `runEmbeddedPiAgent()`. The parameters (`messageTo` and `messageThreadId`) are already defined in the function signature, already resolved from `resolvedDelivery`, and already used consistently throughout the file in other delivery paths. This pattern exactly matches the established pattern in `agent-runner-execution.ts:264-265`. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->