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/ai/agent/conversation.rs

@@ -3320,7 +3320,28 @@ impl AIConversation {
     pub fn cache_miss_tokens(&self) -> u32 {
         self.total_token_usage_by_model
             .values()
-            .map(|u| u.total_input)
+            .map(|u| u.total_input.saturating_sub(u.input_cache_read + u.input_cache_write))
+            .sum()
+    }
+
+    pub fn last_block_cache_read_tokens(&self) -> u32 {
+        self.last_block_token_usage_by_model
+            .values()
+            .map(|u| u.input_cache_read)
+            .sum()
+    }
+
+    pub fn last_block_cache_write_tokens(&self) -> u32 {
+        self.last_block_token_usage_by_model
+            .values()
+            .map(|u| u.input_cache_write)
+            .sum()
+    }
+
+    pub fn last_block_cache_miss_tokens(&self) -> u32 {
+        self.last_block_token_usage_by_model
+            .values()
+            .map(|u| u.total_input.saturating_sub(u.input_cache_read + u.input_cache_write))
             .sum()
     }