Fix critical bugs: JSON crash, data loss, thread safety, security vulnerabilities

[618dt]

Bug Fixes

Critical

• agent_loop.py: Fix JSONDecodeError crash when LLM returns malformed tool arguments
• ga.py: Fix file_write prepend mode data loss on write failure (atomic write via tmpfile+rename)
• ga.py: Sandboxed inline_eval - remove history exposure, restrict builtins
• ga_ultraplan.py: Add Bearer token auth to /exec HTTP endpoint

High

• agentmain.py: Fix thread safety - use self.lock for is_running/stop_sig mutations
• llmcore.py: Fix tryparse unhandled exception on final fallback

Medium

• agentmain.py: Fix 7 file handle leaks (open without with)
• ga.py: Fix file handle leaks in code_run, _check_plan_completion