✨ Configurable default language via env var and ~/.askcc/config.toml

[monkut]

Closes #92

Summary

• Default --language now resolves from CLI > env (ASKCC_LANGUAGE) > ~/.askcc/config.toml [defaults].language > built-in default (english). Invalid values warn and fall through; missing config files are silent.
• Adds _load_user_config() (caches parsed ~/.askcc/config.toml at module import) and _resolve_default_language() in askcc/settings.py, mirroring the existing _resolve_effort_level() pattern.
• Moves SupportedLanguage from askcc/definitions.py to askcc/settings.py to avoid a circular import (same constraint that already pinned VALID_EFFORT_LEVELS here). All known call sites updated.

Files touched

• askcc/settings.py — tomllib import, SupportedLanguage (moved here), USER_CONFIG_PATH, _load_user_config, _USER_CONFIG, _resolve_default_language, DEFAULT_LANGUAGE.
• askcc/definitions.py — remove SupportedLanguage (now lives in settings.py).
• askcc/cli.py — split SupportedLanguage import to .settings; switch argparse default to settings.DEFAULT_LANGUAGE; update help text.
• tests/test_askcc.py — update SupportedLanguage import path; add TestDefaultLanguageResolution (11 tests).
• README.md — document ASKCC_LANGUAGE and ~/.askcc/config.toml precedence.

Test plan

• uv run pytest — 197 passed (11 new tests in TestDefaultLanguageResolution)
• uv run ruff check — clean
• uv run pyright — 0 errors

Risks / notes

• SupportedLanguage import path change is internal-only (package marked Private :: Do Not Upload).
• TOML parse-error log message intentionally omits the exception detail to avoid echoing config contents into logs.
• Module-level _USER_CONFIG cache means refresh requires a fresh process; tests inject a dict via the optional user_config argument.