RLM: Make FIFO IO non-blocking

[snimu]

Description

• Use non‑blocking FIFO read/write with select in local and sandbox workers.
• Fail fast when the worker is dead/missing instead of hanging until timeout.
• Preserve existing timeout/recovery behavior for REPL execution.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• [ x I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Note

Medium Risk
Touches core REPL execution I/O and timeout handling; subtle FIFO/select edge cases could cause new timeouts or incomplete reads/writes across local and sandbox backends.

Overview
Improves RLM REPL worker communication by switching FIFO reads/writes to non-blocking os.open/os.read/os.write with select, instead of blocking file I/O that could hang.

Both local and sandbox execution paths now fail fast when FIFOs/worker are missing or unresponsive, mapping WORKER_DEAD/WORKER_TIMEOUT conditions to RLMCodeExecutionTimeout and preserving the existing timeout recovery path (including FIFO unblocking) with clearer logging.

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