Add activity log

[KTSCode]

Summary

• New Severance.ActivityLog module tracks daemon sessions (start/stop with duration) and overtime protocol usage
• Plain-text log file at ~/.local/state/severance/activity.log (configurable via log_file in config)
• New sev log command prints the activity log to stdout
• Countdown GenServer logs overtime activation and session stop (with duration) via terminate/2

Test plan

• ActivityLog.format_entry/2 formats started, overtime, and stopped entries correctly
• ActivityLog.log_started/1 creates directory, appends entry, stores start time
• ActivityLog.log_overtime/1 appends overtime entry
• ActivityLog.log_stopped/1 appends stopped entry with duration, handles missing start time
• Config.defaults/0 includes log_file key
• Config.generate_contents/1 round-trips with log_file
• Application.resolve_config/2 resolves log_file from config file, expands tilde, stores in env
• Countdown.overtime/0 logs overtime event to activity log
• Countdown.terminate/2 logs stopped event with duration
• CLI.parse_args(["log"]) returns :log
• CLI.run_log/1 prints log contents or "no log found" message
• Full quality suite passes (format, compile, credo, dialyzer, 240 tests, 59.3% coverage)

Implementation Plan

8 tasks executed via subagent-driven development:

1. ActivityLog module with format_entry/2 and default_log_file/0
2. ActivityLog file I/O (log_started, log_overtime, log_stopped)
3. Config — add log_file key to defaults and generate_contents
4. Application — resolve log_file from config, call log_started on daemon boot
5. Countdown — log overtime on activation, log stopped on terminate
6. CLI — sev log command (parse_args, run_log, dispatch)
7. Documentation — README usage, config example, CHANGELOG
8. Full quality check