feat: export Coder Agents debug logs (#25039)

Adds JSON export actions to the Coder Agents Debug panel so users can download either the current chat's recent debug runs or one expanded run for support sharing.

The export reuses the existing chat debug endpoints and react-query cache, adds Storybook and unit coverage for the JSON envelope, and updates the chat debug logging docs with UI and cURL instructions.

Refs CODAGT-280.

Generated by Coder Agents.

<details>
<summary>Implementation notes</summary>

- Chat-level export fetches full detail for each listed debug run with `queryClient.fetchQuery(chatDebugRun(chatId, run.id))` and writes a single JSON file.
- Run-level export uses the already-loaded detail query data from an expanded run card.
- The JSON envelope includes `version`, `scope`, `exported_at`, `chat_id`, and either `runs` or `run`.
- The chat-level export reflects the current backend list endpoint behavior, up to the 100 newest debug runs.
- Agent-browser dogfooding verified files were downloaded and that `jq` validated the chat-level and run-level JSON contents.

</details>

docs/ai-coder/agents/platform-controls/experiments.md

@@ -89,12 +89,83 @@ that chat's owner. The tab lists recent debug runs and lets you expand a run
 into its per-step request, response, token usage, retry attempts, errors,
 and policy metadata.
 
+### Export debug logs
+
+You can export the same captured debug data from the UI:
+
+1. Navigate to **Agents**.
+1. Open a chat with debug logging enabled.
+1. Open the **Debug** tab in the right panel.
+1. Click **Export debug logs** to download the chat's recent debug runs as
+   JSON, or expand a run and click **Export this run** to download one run.
+
+The chat-level export includes the full run detail for the runs returned by
+the debug run list endpoint. The current list endpoint returns up to 100 of
+the newest runs.
+
+### API access
+
 The same data is available through the experimental API:
 
-- `GET /api/experimental/chats/{chat}/runs` lists the most recent runs for a
-  chat (up to 100, newest first).
-- `GET /api/experimental/chats/{chat}/runs/{debugRun}` returns a single run
-  with all of its steps, including normalized request and response bodies.
+- `GET /api/experimental/chats/{chat}/debug/runs` lists the most recent runs
+  for a chat (up to 100, newest first).
+- `GET /api/experimental/chats/{chat}/debug/runs/{debugRun}` returns a single
+  run with all of its steps, including normalized request and response bodies.
+
+Fetch a single run and save it as JSON:
+
+```sh
+export CODER_URL="https://coder.example.com"
+export CODER_SESSION_TOKEN="$(coder login token)"
+export CHAT_ID="00000000-0000-0000-0000-000000000000"
+export RUN_ID="11111111-1111-1111-1111-111111111111"
+
+curl -fsS \
+  -H "Coder-Session-Token: $CODER_SESSION_TOKEN" \
+  "$CODER_URL/api/experimental/chats/$CHAT_ID/debug/runs/$RUN_ID" \
+  | jq . > "coder-agents-debug-run-$RUN_ID.json"
+```
+
+Fetch every run returned by the list endpoint and save a chat-level export.
+Using the same `CODER_URL`, `CODER_SESSION_TOKEN`, and `CHAT_ID` variables
+from above:
+
+```sh
+RUN_IDS=$(curl -fsS \
+  -H "Coder-Session-Token: $CODER_SESSION_TOKEN" \
+  "$CODER_URL/api/experimental/chats/$CHAT_ID/debug/runs" \
+  | jq -r '.[].id') || {
+  echo "Failed to list debug runs" >&2
+  exit 1
+}
+
+RUN_EXPORTS=$(mktemp)
+trap 'rm -f "$RUN_EXPORTS"' EXIT
+
+for RUN_ID in $RUN_IDS; do
+  curl -fsS \
+    -H "Coder-Session-Token: $CODER_SESSION_TOKEN" \
+    "$CODER_URL/api/experimental/chats/$CHAT_ID/debug/runs/$RUN_ID" \
+    >> "$RUN_EXPORTS" || {
+      echo "Failed to fetch debug run $RUN_ID" >&2
+      exit 1
+    }
+  echo >> "$RUN_EXPORTS"
+done
+
+jq -s \
+  --arg chat_id "$CHAT_ID" \
+  --arg exported_at "$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+  '{
+    version: 1,
+    scope: "chat",
+    exported_at: $exported_at,
+    chat_id: $chat_id,
+    run_count: length,
+    limited_to_most_recent: 100,
+    runs: .
+  }' "$RUN_EXPORTS" > "coder-agents-debug-chat-$CHAT_ID.json"
+```
 
 Debug runs are stored alongside the chat and are removed when the parent
 conversation is deleted (manually, by retention, or by chat purge). See