#11296: fix: add sessionKey alias to sessions_spawn for backward compatibility

## Problem Cron jobs using spawn-worker.js were failing with 'No sessionKey in spawn result' because the sessions_spawn tool returns childSessionKey but callers expect sessionKey. ## Solution Added sessionKey as an alias for childSessionKey in the sessions_spawn tool result. This ensures backward compatibility with existing callers while maintaining the more descriptive childSessionKey name. ## Testing - Local testing confirmed the fix resolves cron spawn failures - Both sessionKey and childSessionKey are now available in the result ## Related This complements the local fix in spawn-worker.js that now accepts either field name for maximum compatibility. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a backward-compatible `sessionKey` field to the `sessions_spawn` tool JSON response as an alias for `childSessionKey`. This restores compatibility with existing callers (e.g., cron/spawn-worker flows) that still expect `sessionKey`, while keeping the more explicit `childSessionKey` available. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is a single additive field in the tool response (`sessionKey` aliasing `childSessionKey`) and does not alter control flow or behavior of session spawning; it only improves compatibility for existing consumers. - 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 -->