fix(tui): Container change detection for proper differential rendering

Fixed rendering artifact where duplicate bottom borders appeared when components
dynamically shifted positions (e.g., Ctrl+C in agent clearing status container).

Root cause: Container wasn't reporting as "changed" when cleared (0 children),
causing differential renderer to skip re-rendering that area.

Solution: Container now tracks previousChildCount and reports changed when
child count changes, ensuring proper re-rendering when containers are cleared.

- Added comprehensive test reproducing the layout shift artifact
- Fixed Container to track and report child count changes
- All tests pass including new layout shift artifact test

todos/todos.md

@@ -1,7 +1,3 @@
-- agent/tui: broken rendering of resumed session messages
-    - start session, "read all README.md files except in node_modules
-    - stop session
-    - resume session, messages are cut off?
 
 - pods: pi start outputs all models that can be run on the pod. however, it doesn't check the vllm version. e.g. gpt-oss can only run via vllm+gpt-oss. glm4.5 can only run on vllm nightly.
 
@@ -41,8 +37,6 @@
 
 - agent: test for basic functionality, including thinking, completions & responses API support for all the known providers and their endpoints.
 
-- agent: token usage gets overwritten with each message that has usage data. however, if the latest data doesn't have a specific usage field, we record undefined i think? also,   {"type":"token_usage" "inputTokens":240,"outputTokens":35,"totalTokens":275,"cacheReadTokens":0,"cacheWriteTokens":0} doesn't contain reasoningToken? do we lack initialization? See case "token_usage": in renderers. probably need to check if lastXXX > current and use lastXXX.
-
 - agent: groq responses api throws on second message
     ```
     ➜  pi-mono git:(main) ✗ npx tsx packages/agent/src/cli.ts --base-url https://api.groq.com/openai/v1 --api-key $GROQ_API_KEY --model openai/gpt-oss-120b --api responses