Problem: add_column_if_missing() checks PRAGMA table_info for column existence, then issues ALTER TABLE ADD COLUMN if not found. When two concurrent processes both pass the check before either executes the ALTER, the second process fails with a 'duplicate column name' error.
Fix: Catch the 'duplicate column name' SQLite error after the ALTER TABLE and treat it as a benign no-op. Also explicitly drop statement/rows handles before ALTER to release locks.
Ref: #710 (Item 8)
- Added `JobType`, `SessionTarget`, `Schedule`, `DeliveryConfig`, `CronJob`, `CronRun`, and `CronJobPatch` types in `src/cron/types.rs` for cron job configuration and management.
- Introduced `CronAddTool`, `CronListTool`, `CronRemoveTool`, `CronRunTool`, `CronRunsTool`, and `CronUpdateTool` in `src/tools` for adding, listing, removing, running, and updating cron jobs.
- Updated the `run` function in `src/daemon/mod.rs` to conditionally start the scheduler based on the cron configuration.
- Modified command-line argument parsing in `src/lib.rs` and `src/main.rs` to support new cron job commands.
- Enhanced the onboarding wizard in `src/onboard/wizard.rs` to include cron configuration.
- Added tests for cron job tools to ensure functionality and error handling.
