Validate task numbering/duplication

[deepfriedmind]

After some back and forth when running the proposal command, I ended up with some duplicated tasks and incorrect numbering, e.g.:

- [ ] 1.1 …
- [ ] 1.2 …
- [ ] 1.2 …
- [ ] 1.3 …
- [ ] 1.5 …

It would be nice if this could be caught in validation.

[clay-good]

Moving this to backlog for now. Current validation already reports duplicate task IDs and mismatched numbered groups through the work described in #1520. Numbering gaps are intentionally allowed so removing a task does not force renumbering or break references; ungrouped task lists are also deliberately left alone. Expanding those rules would need a separate design decision rather than a routine bug fix.