Intent: make query-holds KB scoping strict by default

Fix remaining Horn KB scoping gap in intent query-holds.

Current behavior still leaks clauses when a file has multiple kb declarations and --kb is omitted (it queries all KBs).

Required behavior:

• If file has exactly one KB and --kb omitted: use that KB (backward-compatible).
• If file has multiple KBs and --kb omitted: fail fast with clear error and non-zero exit.
• If --kb is provided but no matching KB hash exists: fail fast with clear error and non-zero exit.
• Do not silently return false for missing KB selection errors.

Also:

• Update usage/help text to document strict behavior and --kb option.
• Add regression coverage (and/or unit tests) for all three cases above.
• Preserve current passing behavior for explicitly scoped queries.