docs(task): clarify --due is a server-side pass-through; steer to quickadd

[engfragui]

Background

The --due value on task add / task update is sent verbatim as due_string to the Backend API (the CLI does not parse or rewrite it), i.e. does not handle starting <date> clauses as the user might expect: strings like every! 2 weeks starting 2026-05-17 end up stored as the entire literal recurrence rule and the task never advances on completion.

However note that the quick-add parser does consume starting correctly/as the user expects.

This PR

Document this in three places users actually hit:

• task add --due help: shorter description + a Notes block on td task add --help describing the failure mode and pointing to td task quickadd as the right command for NL recurrence
• task update --due help: same pass-through note, referring back to td task add --help
• Skill content (src/lib/skills/content.ts) — adds a bullet to the existing "Choosing between task add and task quickadd" guidance. Regenerated skills/todoist-cli/SKILL.md via npm run sync:skill

Reference

Helps with #342

[doistbot]

Thank you for adding this helpful documentation to clarify the --due parsing limitations and steering users toward the natural language capabilities of quickadd. This update will significantly reduce user frustration around complex recurrence rules. A couple of adjustments were flagged during review, specifically regarding the need to scope the quickadd recommendation strictly to task creation rather than updates, as well as the need for test coverage of the help output in task.test.ts to ensure the new guidance is preserved.

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[engfragui]

@scottlovegrove Apologies, I keep forgetting we automatically add a review right away to PRs in this repo, so I guess it tagged you when I was still working on the PR (edit: it actually tagged Ernesto and you were just a lurker!). Either way, I'm done now (just did a little re-wording, nothing much).