fix(skills_guard): explain why --force is rejected on dangerous verdicts

Follow-up to @sprmn24's verdict-logic fix. The previous block-message
ended in 'Use --force to override' regardless of verdict — but as of
the --force fix above, dangerous community/trusted skills can't be
overridden by --force at all. The misleading hint sends users in a
loop. Replace it with a specific message that tells them what the
documented behavior actually is.

Adds two regression tests covering the dangerous-verdict message
shape and one that pins the existing --force hint for non-dangerous
blocks.

tools/skills_guard.py

@@ -674,6 +674,13 @@ def should_allow_install(result: ScanResult, force: bool = False) -> Tuple[bool,
             f"{len(result.findings)} findings)"
         )
 
+    # Dangerous verdicts cannot be overridden by --force (community/trusted);
+    # other blocks can.
+    if result.verdict == "dangerous" and result.trust_level in ("community", "trusted"):
+        return False, (
+            f"Blocked ({result.trust_level} source + dangerous verdict, "
+            f"{len(result.findings)} findings). --force does not override a dangerous verdict."
+        )
     return False, (
         f"Blocked ({result.trust_level} source + {result.verdict} verdict, "
         f"{len(result.findings)} findings). Use --force to override."