OIC UI: investigation workspace #229

Context

Users must be able to run an investigation and answer the agent's questions without opening a terminal. This lives in the same app as the review-audit UI (#167) and uses only the API (#228).

Behavior to implement

• Start an investigation: an objective in plain language and an optional scope (team, repo, service).
• Workspace: title, objective, scope, a clear status (running, waiting for you, waiting for approval, completed, failed), a timeline of messages and agent activity, evidence and artifacts collected, proposed changes, errors and retries.
• Questions from the agent are shown prominently and answered in the same thread.
• After completion: what changed in the knowledge.

Acceptance criteria

• Create an investigation, answer a question, and watch it resume and complete, all in the browser.
• Closing the browser mid-run and reopening shows the current state.