[feature] quest update: support clearing all dependencies via --clear-depends-on flag

[kunallanjewar]

Summary

Today guild quest update --depends-on "" is ambiguous — empty-string is treated inconsistently between the CLI and MCP surfaces, and there's no dedicated "drop all dependencies" verb. Agents reach for --replace-depends-on "" or similar and it silently no-ops.

Acceptance

• Add --clear-depends-on boolean flag to guild quest update that removes all dependency edges for the target quest.
• MCP tool quest_update exposes the same capability (e.g. clear_depends_on=true kwarg).
• --depends-on "" with --replace-depends-on also clears (for symmetry), or produces a clear error directing the user to --clear-depends-on.
• Unit test covers: before=2 deps → after=0 deps.
• Help text for quest update documents the new flag with a one-line example.

Not in scope

• Reworking the storage representation of dependencies.
• Adding per-edge removal (dropping a single dep) — already supported via --replace-depends-on.