fix(scheduled): distinguish hourly placeholders from calendar times

[hui455]

Changing an Hourly task to Daily or Weekly through AI without supplying a time can silently reuse the internal 00:00 placeholder as a calendar setting.

Require an explicit calendar time unless the task already has a known Daily/Weekly configuration. Existing calendar settings, including midnight, survive an Hourly round trip and restart. Older Hourly tasks keep their data but need a time when converting, since the original intent cannot be recovered reliably.

Before and after — actual Windows desktop (1200 × 800, isolated profiles). A deterministic local model fixture calls the real tools and summarizes their results; no live model is used.

Before:

After:

Regression tests reproduce the implicit conversion and cover explicit midnight, saved calendars, paused state and restart. The optional configuration marker uses the existing JSON storage; no table migration is needed.

Validation details

• Both calendar regression tests and real Host conversion/restart E2E passed. The Manual-to-Hourly compatibility E2E passed.
• Scheduled Rust suite: 20 passed, 4 existing Windows path assertion failures. The separate workspace-path fix addresses those assertions.
• Formatting and diff checks passed.
• Windows, Node 24.2, Rust debug build. Native macOS/Linux runs were not performed. Screenshots use main 0111e306c and the corresponding fix; focused checks were rerun after refreshing the candidates to 46d4ee4ee.

[vastsa]

Landing note: this branch conflicted with the sibling scheduled-task PRs (#820-#824) that landed just before it. I merged the updated main into this branch and resolved the conflicts (docs sections unioned; for #822 the new tests in scheduled_tools.rs were combined with #820's). No functional change was made to your commits.