Port missing features to gui_2 and optimize caching

- Port 10 missing features from gui.py to gui_2.py: performance
    diagnostics, prior session log viewing, token budget visualization,
    agent tools config, API hooks server, GUI task queue, discussion
    truncation, THINKING/LIVE indicators, event subscriptions, and
    session usage tracking
  - Persist window visibility state in config.toml
  - Fix Gemini cache invalidation by separating discussion history
    from cached context (use MD5 hash instead of built-in hash)
  - Add cost optimizations: tool output truncation at source, proactive
    history trimming at 40%, summary_only support in aggregate.run()
  - Add cleanup() for destroying API caches on exit

aggregate.py

@@ -164,6 +164,18 @@ def build_markdown_from_items(file_items: list[dict], screenshot_base_dir: Path,
     return "\n\n---\n\n".join(parts)
 
 
+def build_markdown_no_history(file_items: list[dict], screenshot_base_dir: Path, screenshots: list[str], summary_only: bool = False) -> str:
+    """Build markdown with only files + screenshots (no history). Used for stable caching."""
+    return build_markdown_from_items(file_items, screenshot_base_dir, screenshots, history=[], summary_only=summary_only)
+
+
+def build_discussion_text(history: list[str]) -> str:
+    """Build just the discussion history section text. Returns empty string if no history."""
+    if not history:
+        return ""
+    return "## Discussion History\n\n" + build_discussion_section(history)
+
+
 def build_markdown(base_dir: Path, files: list[str], screenshot_base_dir: Path, screenshots: list[str], history: list[str], summary_only: bool = False) -> str:
     parts = []
     # STATIC PREFIX: Files and Screenshots must go first to maximize Cache Hits
@@ -195,8 +207,9 @@ def run(config: dict) -> tuple[str, Path, list[dict]]:
     output_file = output_dir / f"{namespace}_{increment:03d}.md"
     # Build file items once, then construct markdown from them (avoids double I/O)
     file_items = build_file_items(base_dir, files)
+    summary_only = config.get("project", {}).get("summary_only", False)
     markdown = build_markdown_from_items(file_items, screenshot_base_dir, screenshots, history,
-                                         summary_only=False)
+                                         summary_only=summary_only)
     output_file.write_text(markdown, encoding="utf-8")
     return markdown, output_file, file_items