tracing: trace waterfall + summary prompt + chat tab all lose data after each agent turn

[sahrizvi]

The trace UI (/traces in the TUI) drops most of the conversation data after every agent turn. Three observable symptoms in a normal multi-turn session on altimate-default:

1. Waterfall view collapses to the system-prompt span after each agent turn. Real session has many tool calls and generations; trace file on disk has 1 root span and metadata: {} once the turn completes.
2. Summary tab shows "What was asked / No prompt recorded" even when a real user prompt was typed.
3. Chat tab shows only the last user message in multi-turn sessions. Earlier user turns render as their assistant responses but the user's "▶ You" bubbles are missing.

Repro

Open a TUI session, send a couple of messages with tool-using replies (e.g. "list directories", then "okay, how about patches"), open /traces, watch the file:

ls -la ~/.local/share/altimate-code/traces/ses_<id>.json
jq '{spans: (.spans | length), metadata, summary}' ~/.local/share/altimate-code/traces/ses_<id>.json

Before the fix: spans: 1, metadata: {}, summary counters at 0/null, regardless of how much activity the DB shows for the session. The viewer's chat tab shows only one ▶ You bubble for the latest prompt.

Scope

Closed by the fix in this PR. Filing for traceability so the merge cleanly closes a tracked symptom report.