fix: clarify identity requirements for SendMessage, remove misleading investigation fallback

[tipuq]

Problem

Two doc statements sent users down rabbit holes:

1. The SendMessage API reference implied userId is "optional for investigation executionIds" — in practice, both CreateChat and SendMessage require identity resolution. If one fails, the other will too.

2. Multiple places suggested using SendMessage on investigation executionIds as a fallback when chat identity fails. This doesn't work with current service behavior and wastes debugging time.

Changes

POWER.md (4 targeted replacements):

• SendMessage row: "may be optional for investigation executionIds" → "userId is always required"
• Setup note: "you can still use SendMessage on investigation executionIds" → "SendMessage will fail the same way — use the investigation workflow instead"
• Follow-up section: "You can use SendMessage with an investigation's executionId" → "Use list-journal-records. Do NOT use SendMessage with investigation executionIds"
• Troubleshooting: "SendMessage on investigation executionIds may work without userId" → "use the investigation workflow which does not require user identity"

steering.md (1 replacement):

• Error handling: removes "Fallback: SendMessage on investigation executionIds may work without userId"

The correct mental model

• Chat (CreateChat + SendMessage): requires identity resolution. If it fails, chat is unavailable.
• Investigations (create-backlog-task + list-journal-records): does NOT require identity. Always works.
• These are separate workflows — do not cross executionIds between them.