RLM: make bash REPL default, keep Python REPL optional

[snimu]

Description

Overview:

Adds a Bash REPL mode to RLMEnv (default), while keeping Python mode unchanged. Improves root-tool calling in Bash with --json support (args/kwargs schema) and JSON stdin handling.

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
• 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

---

Note

Makes RLMEnv Bash-first with configurable REPL, new worker/tool plumbing, and resilience improvements.

• Default REPL = Bash via repl_language ("bash" default, "python" optional); registers call_bash_repl or call_python_repl accordingly; system prompt added for Bash
• Bash worker + tool helper: new worker script template and root-tool helper supporting --json, stdin payloads, positional args, and llm_batch output headers; root tools callable as shell commands
• Executor/IO changes: uses ThreadPoolExecutor for FIFO IO; best-effort unblocks response FIFO on timeouts; skips venv creation in Bash mode; renders worker script based on repl_language; cleans up IO executor on teardown
• Output/UX: Bash REPL returns raw combined output; context-limit warnings shown in both modes; root tools documentation includes Bash usage; Python package docs suppressed in Bash mode
• API/docs/tests: adds repl_language param and updates docs (environments.md, experimental README); broad new tests for Bash prompt/output, worker rendering, tool helper behaviors, and context warnings

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