Feature/one shot flag

[code-quad3]

Add --one-shot flag for sessionless single-prompt execution

Closes #5143

Summary

Adds a --one-shot flag for running a single prompt without git or session context, then exiting. Useful for quick lookups, code analysis, and scripting use cases.

Usage

# Simple prompt
aider --one-shot "what is 2+2"
Explain a file
aider --one-shot "explain this file" src/utils.py
Modify a file
aider --one-shot "add type hints" src/utils.py --one-shot-mode code

Behavior

• Implies --no-git — no repository initialization
• Non-interactive — no prompts, auto-accepts all actions
• Disables repo map
• Exits with code 0 on success, 1 on error
• Default mode is ask (read-only); use --one-shot-mode code to apply edits

New Flags

Flag	Description
--one-shot PROMPT	Run a single prompt and exit
--one-shot-mode ask|code	Mode to use (default: ask)

Changes

• aider/args.py — added --one-shot and --one-shot-mode arguments
• aider/main.py — early block sets git/session defaults; handler block runs prompt and exits
• tests/basic/test_main.py — 5 new tests covering no-git, prompt execution, exit behavior, and file context in both modes

Tests

tests/basic/test_main.py::TestMain::test_one_shot_no_git                   PASSED
tests/basic/test_main.py::TestMain::test_one_shot_runs_prompt              PASSED
tests/basic/test_main.py::TestMain::test_one_shot_exits_after_response     PASSED
tests/basic/test_main.py::TestMain::test_one_shot_ask_mode_files_readonly  PASSED
tests/basic/test_main.py::TestMain::test_one_shot_code_mode_files_editable PASSED

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