feat: track step runtime_ms on chat messages (#23219)

## Summary

Adds a `runtime_ms` column to `chat_messages` that records the
wall-clock duration (in milliseconds) of each LLM step. This covers LLM
streaming, tool execution, and retries — the full time the agent is
"alive" for a step.

This is the foundation for billing by agent alive time. The column
follows the same pattern as `total_cost_micros`: stored per assistant
message, aggregatable with `SUM()` over time periods by user.

## Changes

- **Migration**: adds nullable `runtime_ms bigint` to `chat_messages`.
- **chatloop**: adds `Runtime time.Duration` field to `PersistedStep`,
measures `time.Since(stepStart)` at the beginning of each step (covering
stream + tool execution + retries).
- **chatd**: passes `step.Runtime.Milliseconds()` to the assistant
message `InsertChatMessage` call; all other message types (system, user,
tool) get `NULL`.
- **Tests**: adds `runtime > 0` assertion in chatloop tests.

## Billing query pattern

Once ready, aggregation mirrors the existing cost queries:

```sql
SELECT COALESCE(SUM(cm.runtime_ms), 0)::bigint AS total_runtime_ms
FROM chat_messages cm
JOIN chats c ON c.id = cm.chat_id
WHERE c.owner_id = @user_id
  AND cm.created_at >= @start_time
  AND cm.created_at < @end_time
  AND cm.runtime_ms IS NOT NULL;
```

coderd/chatd/chatloop/chatloop_test.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
 	"charm.land/fantasy"
 	fantasyanthropic "charm.land/fantasy/providers/anthropic"
@@ -64,6 +65,8 @@ func TestRun_ActiveToolsPrepareBehavior(t *testing.T) {
 	require.Equal(t, 1, persistStepCalls)
 	require.True(t, persistedStep.ContextLimit.Valid)
 	require.Equal(t, int64(4096), persistedStep.ContextLimit.Int64)
+	require.Greater(t, persistedStep.Runtime, time.Duration(0),
+		"step runtime should be positive")
 
 	require.NotEmpty(t, capturedCall.Prompt)
 	require.False(t, containsPromptSentinel(capturedCall.Prompt))