fix: add AIDER_CHAT_MODE env var for --chat-mode alias (issue #5367)

[chrislazar25]

Summary: Add AIDER_CHAT_MODE env var support for the --chat-mode / --edit-format argument. Previously only AIDER_EDIT_FORMAT was recognized (auto-derived from --edit-format), but the --chat-mode alias was missing its own env var.

Technical Details:

• Root Cause: configargparse.auto_env_var_prefix derives env var names from the first option string only. For --edit-format / --chat-mode, only AIDER_EDIT_FORMAT is auto-generated; AIDER_CHAT_MODE is not.
• Mechanism: Set action.env_var = "AIDER_CHAT_MODE" on the argument after parser creation. For backward compatibility, also inject AIDER_CHAT_MODE from AIDER_EDIT_FORMAT if only the latter is set. Priority: CLI > AIDER_CHAT_MODE > AIDER_EDIT_FORMAT > config file > default.

Verification: Confirmed adherence to CONTRIBUTING.md. Ran python -m pytest tests/. 455 passed, 1 pre-existing env failure (test_voice requires audio hardware). Pre-commit hooks (isort, black, flake8, codespell) all green. Manual E2E:

• AIDER_CHAT_MODE=ask → edit_format=ask
• AIDER_EDIT_FORMAT=architect → edit_format=architect (backward compat)
• CLI --chat-mode ask + env var → CLI wins

Blast Radius:

• Impact: Low. Adds a new env var. Existing AIDER_EDIT_FORMAT usage continues to work.
• Trade-offs: When both AIDER_CHAT_MODE and AIDER_EDIT_FORMAT are set, AIDER_CHAT_MODE takes priority.

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Fixes #5367