feat(observability): /trace should walk parent_agent_id chain

[justrach]

Background

The `/trace [N|all]` slash command (added in #28) calls `API::list_trajectory(conversation_id)`, which queries:

```sql
SELECT * FROM trajectory_events
WHERE conversation_id = ? AND agent_id = <active_agent>
ORDER BY seq;
```

So `/trace` shows only the active agent's trajectory. When the active agent is the root and it spawned subagents (under different `agent_id`s), those subagent rows are invisible. To inspect them today you'd have to know each child's agent_id and run a separate query.

Proposed fix

Two pieces:

1. Repo: walk children. Add `TrajectoryRepo::list_with_children(conv, root_agent) -> Vec` that recursively pulls everything where `parent_agent_id` chains back to `root_agent`. Already supported by the `(conversation_id, agent_id, seq)` index — one query per level.

2. Renderer: indent by depth. Update the /trace renderer in `crates/forge_main/src/ui.rs::on_trace` to indent child events under their parent's row, so a tree like:

```
1 call Task call_id=t1
12 call read call_id=c1 (child agent A, indented)
13 result read duration=4ms
2 result Task duration=120ms
```

… makes the parent → child structure visible.

Acceptance

• `/trace` of a conversation with subagents shows parent + child events in one view
• Indentation makes the parent_agent_id relationship clear at a glance
• `/trace all` still works on conversations with no subagents (no regression)

Depends on

• feat(observability): record subagent (Task tool) trajectories #30 (subagents must be recorded for there to be anything to walk)

[justrach]

Shipped. Added TrajectoryRepo::list_for_conversation (single SELECT across all agents). API::list_trajectory now uses it; /trace renders depth via parent_agent_id walking and indents children under their parents.