fix: clear cluster dependencies

[ApexWorm]

Problem

desloppify plan cluster update --depends-on requires at least one cluster, so a stale dependency edge cannot be removed through the CLI. Passing an empty value becomes an invalid cluster name instead of clearing the list.

Fix

Add an explicit mutually exclusive --clear-depends-on flag. The update flow maps it to an empty dependency list, persists that list, and reports that dependencies were cleared. Existing manually constructed argument namespaces remain compatible.

Validation

• python3 -m pytest -q desloppify/tests/commands/test_parser_groups_option_sections_direct.py desloppify/tests/commands/plan/test_cluster_ops_direct.py (15 passed)
• python3 -m ruff check on changed files
• Full suite: 5,811 passed, 3 skipped; two pre-existing review-prompt assertions fail identically on clean origin/main.