refactor: unify agent history and harden runtime diagnostics

Preserve user guidance through history compression, stabilize Flash index targets, and retire withdrawn checks without applying stale verdicts.

Improve runtime diagnostics, tool responses, and history rendering. Simplify documentation and comments.

Validation: 2076 tests passed in the full suite; 381 affected tests and 26 frontend tests passed after cleanup. Ruff and the quality ratchet passed.

scripts/run_ui.py

@@ -17,7 +17,7 @@
 
 Fastest and simplest way to launch the Artemis Web UI across all platforms:
 - Automatically detects if the UI is already running and opens the browser directly.
-- Uses uv or existing virtualenv seamlessly.
+- Uses uv or an existing virtualenv.
 - Zero-configuration one-click start.
 """