bug(versions): links --type errors never list the valid values

Summary

asc versions links fails for 77.5% of installs (74 distinct installs, 5.x, 14 days): 55 hit missing_required on --type, 18 hit invalid_value on --type, 18 hit 404. Neither error enumerates the accepted --type values.

Proposed fix

• Enumerate the valid --type values in both the missing-required and invalid-value errors and in the flag help.
• Consider a sensible default if one link type dominates usage; otherwise keep it required.
• Make the 404 path say which resource was not found (version vs link).

Acceptance criteria

• cmdtest: omitted --type → exit 2 with the value list; bad value → exit 2 with the value list
• help text lists the values; docs/COMMANDS.md regenerated if help changed