Add budget thresholds and automation-friendly exit codes

[SuvenSeo]

Goal

Allow users to set local daily, weekly, monthly, or per-project budget thresholds and return non-zero exit codes when thresholds are exceeded.

Why

This would let users wire the tracker into scheduled tasks, CI-like checks, or local reminders.

Acceptance Criteria

• Users can set threshold values by CLI flags or config file.
• Exceeded thresholds print a clear summary.
• Exit codes are documented and tested.
• No network calls are required.