Refactor: Split main.rs into smaller modules

[evansenter]

Summary

main.rs is currently ~1500 lines (after TUI removal in PR #86), still above the 500-line guideline. It mixes several concerns:

• CLI parsing and Args struct
• OutputSink trait and implementations (LoggingOutputSink)
• OUTPUT_SINK global state
• Interactive REPL logic

Proposed Split

New Module	Contents	Why
src/cli.rs	CLI parsing, Args struct	Separate concern
src/repl.rs	Interactive REPL logic, builtin commands	Separate concern

OutputSink to logging.rs: Move OutputSink trait, LoggingOutputSink, and OUTPUT_SINK global to logging.rs. This is the right home because:

• OutputSink controls where log_event() output goes
• logging.rs already has log_event() and related functions
• Keeps all logging infrastructure together

Current State

After PR #86 (TUI removal):

• No more TuiSink, TuiEventHandler
• OutputSink implementations: just LoggingOutputSink
• main.rs is cleaner but still too large

Effort

Medium - requires careful extraction to avoid circular dependencies.

Related

• PR refactor: remove TUI mode, simplify to plain REPL #86 - Removed TUI, simplified OutputSink