fix: Randomize bash tool sentinel to prevent output truncation from LLM-generated commands

[Ryuketsukami]

Problem

The bash tool in agent/tools/bash.py appends ; echo '<<exit>>' to every command and waits for this exact string in stdout to detect completion. This is a well-known fragile pattern — if LLM-generated code outputs the literal string <<exit>> (e.g., in test output, debug logging, or string manipulation), the tool prematurely truncates the real output.

In a self-improving agent framework where the meta-agent generates arbitrary code, this is a realistic failure mode.

Solution

Replaced the static sentinel with a per-instance random UUID: <<EXIT_{uuid4().hex}>>. Each BashSession gets a unique sentinel at construction time. Updated filter_error() to match the new <<EXIT_ prefix pattern for stderr cleanup.

Tests

Added tests/test_bash_sentinel.py (6 tests):

• Sentinel is not the old static string
• Matches expected pattern <<EXIT_[hex]>>
• Each instance gets a unique sentinel
• 50 instances all produce unique sentinels
• Sentinel is used in the run command

Question for maintainers

Was the static sentinel chosen for simplicity, or does any downstream parsing depend on the exact <<exit>> string? The change is backward-compatible for command execution — only the detection mechanism is different.

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