Implement local sub-agent execution for OSS/Bedrock mode

- Fix start_agent tool mapping: route to Tool::StartAgent instead of
  dead-end Tool::Subagent so tool calls become executable actions
- Block start_agent until child finishes: parent waits for child
  conversation to complete and receives full output as tool result
- Fix tool result delivery: add StartAgent/StartAgentV2 cases to
  extract_tool_result_content so the model actually sees agent output
- Support parallel agent spawning: change StartAgent action phase from
  Serial to Parallel, and track multiple pending agents via Vec
- Mark child conversations as Success on EndTurn: emit
  ConversationStatus::Success when a child stream ends with no actions
- Skip orchestration SSE in local mode: prevent app freeze from trying
  to connect to non-existent server
- Remove send_message_to_agent and suggest_next_prompt from tool list:
  these require server infrastructure that doesn't exist in OSS mode
- Add "Waiting for sub-agents..." status message while agents process
- Fix child agent pane close: actually dismiss instead of re-hiding,
  track dismissed IDs to prevent re-creation on restart
- Fix cache_miss_tokens calculation and show per-block cache stats
- Add [tool-debug] logging throughout tool invocation pipeline

Bump version to 1.6.0.

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

app/src/terminal/input/agent.rs

@@ -634,9 +634,9 @@ fn render_session_status_bar(appearance: &Appearance, app: &AppContext, conversa
     let (cache_read, cache_write, cache_miss, cost_cents, context_usage, current_context) =
         if let Some(conversation) = BlocklistAIHistoryModel::as_ref(app).conversation(&conversation_id) {
             (
-                conversation.total_cache_read_tokens(),
-                conversation.total_cache_write_tokens(),
-                conversation.cache_miss_tokens(),
+                conversation.last_block_cache_read_tokens(),
+                conversation.last_block_cache_write_tokens(),
+                conversation.last_block_cache_miss_tokens(),
                 conversation.total_cost_cents(),
                 conversation.context_window_usage(),
                 conversation.current_context_tokens(),