Resolve issue #921: Cron jobs missed / unclear schedules; local-TZ and active hours support

[YellowSnnowmann]

Summary

• Standardized cron jobs to use device-local timezone by default when no timezone is specified.
• Introduced ActiveHours support to restrict cron job execution to specific time windows (e.g., 09:00-17:00).
• Enhanced CronAddTool documentation and schema to accurately describe polling behavior and timezone defaults.
• Updated core scheduling logic to correctly handle midnight-spanning active hour windows.

Problem

Cron jobs were defaulting to UTC, which was non-obvious to users expecting local time. Additionally, there was no first-class support for "active hours" (e.g., "only during work hours"), forcing users to write complex cron expressions. The product surface was also opaque about the scheduler's polling nature.

Solution

• Added ActiveHours struct and updated Schedule::Cron variant in src/openhuman/cron/types.rs.
• Updated next_run_for_schedule in src/openhuman/cron/schedule.rs to use chrono::Local as the default and implemented a skipping loop for ActiveHours.
• Updated CronAddTool to expose these new capabilities to the agent/user and provide "honest" product copy.
• Added comprehensive unit tests for local timezone defaults and active hours (including midnight crossing).

Submission Checklist

• Unit tests — Added tests in src/openhuman/cron/schedule.rs and updated ops_tests.rs, scheduler_tests.rs.
• E2E / integration — (Existing unit tests cover the logic thoroughly; no UI changes yet).
• Doc comments — Updated tool descriptions.
• Inline comments — Added comments explaining local TZ standardization and midnight-spanning windows.

Impact

• Migration: Existing jobs without a tz will now be interpreted in local time instead of UTC. This is a intentional breaking change in behavior to align with the personal assistant product goal.
• Performance: Negligible impact; the safety loop for active hours is efficient.

Related

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PR created automatically by Jules for task 10913585179801151017 started by @YellowSnnowmann

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