Add a Causal Lens for errors, answers, and file changes

[raphant]

Goal

Let users select an outcome and see the event chain that produced it.
Useful outcomes include an error, final answer, file edit, commit, or completed subagent result.

Proposed change

Build a causal graph from existing parent, agent, tool, file, and time relationships.
Dim unrelated nodes and highlight the prompts, tools, files, and subagents on the selected path.

Acceptance criteria

• Users can start the lens from an error, answer, edit, or subagent result.
• The graph distinguishes established event links from time-based inferences.
• Each highlighted edge explains why the events are connected.
• Users can clear the lens and return to the unchanged live graph.
• A fixture test covers one branching session with an error and recovery.

Source

• web/hooks/simulation/process-event.ts converts events into graph state.
• web/components/agent-visualizer/canvas/draw-edges.ts renders graph relationships.