Structured Logging + verbose / quiet / json

[LeeSinLiang]

Structured logging + --verbose/--quiet (optional --json)

Summary
Adopt Python logging across the CLI with consistent levels and user-friendly verbosity flags; optional machine-readable JSON logs.

Scope

• Default level INFO; --verbose → DEBUG; --quiet → WARN/ERROR only.
• Optional --json to emit structured logs (one JSON object per line).
• Replace print calls with logger; keep UX-friendly messages by default.

Tasks

• Centralize logger setup (single module).
• Wire flags into all commands.
• Add docs and examples in README.

Acceptance criteria

• All codepaths route through logging; flags affect output consistently.
• No regressions in normal (INFO) UX.

Test plan
Unit test logger config; manual test commitlm generate --verbose shows debug traces.