fix: walkthrough frontmatter + clarify identity requirements for chat

[tipuq]

Changes

1. Walkthrough frontmatter — adds name + description to ecs-incident-walkthrough.md so Kiro can keyword-match and auto-load it (per steering docs).

2. Identity resolution clarity — removes misleading suggestions that SendMessage works on investigation executionIds or without identity resolution:

   • SendMessage API reference: "may be optional" → "userId is always required"
   • Setup note: "you can still use SendMessage on investigation executionIds" → "use the investigation workflow instead"
   • Follow-up section: removed suggestion to SendMessage with investigation executionIds
   • Troubleshooting: "may work without userId" → "use investigation workflow which does not require user identity"
   • Steering: removed "Fallback: SendMessage on investigation executionIds may work without userId"

Why

The walkthrough needs name/description for Kiro's auto-inclusion to keyword-match it (follow-up from PR #127 review).

The identity docs sent users down a rabbit hole — both CreateChat and SendMessage require identity resolution. If one fails, the other will too. Investigations (create-backlog-task + list-journal-records) are the correct fallback since they don't require user identity.