feat(cli,config): add config file, providers/models listing, and help command

[lorecanc]

Summary

Ergonomics and discoverability:

• Config file — optional ~/.config/opencode-usage/config.toml ($XDG_CONFIG_HOME aware) with [defaults] (days/weeks/months, by, sort) and [insights] preferred_models that orders the interactive model picker. Precedence: CLI flags > config > built-in defaults. Missing/malformed files silently fall back to defaults.
• providers / models — list IDs sourced from opencode models (cached per run), so they always match your actual setup. providers marks connected providers (*, from auth.json); models supports --provider (repeatable), --id (bare model IDs as stored in the DB), and --json for both.
• help — opencode-usage help [run|insights|providers|models], plus exhaustive English epilogs on every parser covering config, filters, output modes, and examples.

Changes

• src/opencode_usage/config.py: new — TOML loading via tomllib (stdlib, Python 3.11+; the project requires 3.10+, so this keeps >=3.10 compatibility by importing tomllib directly — note: if you want to support 3.10 strictly, we can switch to tomli for <3.11)
• src/opencode_usage/cli.py: help/providers/models subcommands, config defaults applied in _cmd_run/_cmd_insights, epilogs
• src/opencode_usage/models.py: picker honors preferred_models from config
• tests/test_config.py: new — loading, precedence, malformed files
• tests/test_cli.py, tests/test_models.py: new tests
• README.md: documented

Verification

• uv run pytest tests/ — 385 passed
• uvx ruff check . and uvx ruff format --check . — clean

Note

tomllib requires Python 3.11+. The project declares >=3.10; if 3.10 support matters, happy to use tomli with a fallback import instead.