fix(scheduled): allow maintenance of unarmed legacy tasks

[hui455]

Pausing or renaming an older automatic task through AI fails with a schedule-required error when the record predates calendar configuration. Updating its prompt fails for the same reason.

Allow metadata and pause updates without configuring automatic execution or changing the workspace binding. Repeating an unchanged Hourly cadence during maintenance no longer inserts a placeholder. Explicit arming still validates the schedule, and Manual-to-Hourly keeps its default one-hour interval.

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 rejected pause and cover renaming, prompt edits, echoed cadence, guarded resume and reopening SQLite for Hourly, Daily and Weekly tasks.

Validation details

• Legacy maintenance regressions and real Host import/update/restart/delete E2E passed. The Manual-to-Hourly compatibility E2E passed.
• Scheduled Rust suite: 20 passed, 4 existing Windows path assertion failures, addressed by the separate workspace-path fix.
• 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.