feat(slack): Level 3 — ambient cross-agent context from Slack channels

When a cron fires, the scheduler now fetches the last 10 messages from
the workspace's Slack channel via conversations.history and prepends them
to the cron prompt as '[Slack channel context — recent team messages]'.

This gives each agent ambient awareness of what peers are doing:
- Backend sees Frontend posted 'PR #840 ready for review' → can check
- Security Auditor sees Backend posted 'new endpoint added' → plans review
- PM sees all engineering activity → better synthesis in rollup

Implementation:
- slack.go: FetchChannelHistory() calls conversations.history, filters
  bot's own messages, returns last N as SlackHistoryMessage structs
- manager.go: FetchWorkspaceChannelContext() looks up the workspace's
  Slack config, fetches history, formats as readable context block
- scheduler.go: ChannelBroadcaster interface extended with
  FetchWorkspaceChannelContext; fireSchedule injects context before
  the cron prompt (prepended, not appended, so the agent sees team
  context BEFORE its task instructions)

Best-effort: if Slack API fails or workspace has no channels, the
prompt is unchanged. Truncated to 200 chars per message, 10 messages
max to keep prompt overhead bounded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

platform/internal/scheduler/scheduler.go

@@ -43,9 +43,10 @@ type scheduleRow struct {
 	Prompt      string
 }
 
-// ChannelBroadcaster posts messages to a workspace's configured social channels.
+// ChannelBroadcaster posts messages to and reads context from workspace channels.
 type ChannelBroadcaster interface {
 	BroadcastToWorkspaceChannels(ctx context.Context, workspaceID, text string)
+	FetchWorkspaceChannelContext(ctx context.Context, workspaceID string) string
 }
 
 // Scheduler polls the workspace_schedules table and fires A2A messages
@@ -260,6 +261,17 @@ func (s *Scheduler) fireSchedule(ctx context.Context, sched scheduleRow) {
 	fireCtx, cancel := context.WithTimeout(ctx, fireTimeout)
 	defer cancel()
 
+	// Level 3: inject ambient Slack channel context into the cron prompt.
+	// The agent sees recent peer messages before acting, enabling cross-agent
+	// awareness without explicit A2A delegation. Best-effort — if the fetch
+	// fails or the workspace has no Slack channels, the prompt is unchanged.
+	prompt := sched.Prompt
+	if s.channels != nil {
+		if channelCtx := s.channels.FetchWorkspaceChannelContext(fireCtx, sched.WorkspaceID); channelCtx != "" {
+			prompt = channelCtx + "\n" + prompt
+		}
+	}
+
 	msgID := fmt.Sprintf("cron-%s-%s", short(sched.ID, 8), uuid.New().String()[:8])
 
 	a2aBody, _ := json.Marshal(map[string]interface{}{
@@ -268,7 +280,7 @@ func (s *Scheduler) fireSchedule(ctx context.Context, sched scheduleRow) {
 			"message": map[string]interface{}{
 				"role":      "user",
 				"messageId": msgID,
-				"parts":     []map[string]interface{}{{"kind": "text", "text": sched.Prompt}},
+				"parts":     []map[string]interface{}{{"kind": "text", "text": prompt}},
 			},
 		},
 	})