Add --since and --until flags to mine local command

[bk-simon]

Adds date range filtering to the contextweaver mine local command, allowing users to mine only a specific date range of git history.

Changes

• Added --since and --until CLI options to the mine local command
• Updated orchestrator to pass date filters through to the mining agent
• Implemented date filtering logic in mine_local_git() method
• Dates can be provided in ISO format or YYYY-MM-DD format

Usage

# Mine commits from the last month
contextweaver mine local ./my-repo --since 2026-01-20

# Mine commits within a specific range
contextweaver mine local ./my-repo --since 2026-01-01 --until 2026-02-01

Closes #4