feat(cli,insights): wire --db path override and add --no-llm data-only mode

[lorecanc]

Summary

Two small fixes around the insights pipeline:

1. --db PATH — the insights orchestrator already read args.db (getattr(args, "db", None)), but no parser flag ever set it, so the path could only be overridden via the OPENCODE_DB env var. This wires the flag through both run and insights, and _cmd_run now opens the DB at the requested path.

2. --no-llm — lets you generate a data-only insights report without invoking any LLM (previously only possible implicitly when the LLM phase failed).

Changes

• src/opencode_usage/cli.py: --db on run/insights, --no-llm on insights, OpenCodeDB(args.db) in _cmd_run
• src/opencode_usage/insights/orchestrator.py: skip the LLM phase when no_llm is set, with the existing graceful-degradation path unchanged
• tests/test_cli.py, tests/test_insights_orchestrator.py: new tests (--db parse on both subcommands, --no-llm default/set, LLM phases skipped in data-only mode)

Verification

• uv run pytest tests/ — 377 passed
• uvx ruff check . and uvx ruff format --check . — clean