fix(prompts): classify every sub-8B model (2b/4b/5b/6b) as SMALL

detect_model_size only matched 1b/3b/7b, so a genuinely small model such as
qwen*:4b fell through to LARGE and got the terse, less-guided prompt set it
can't follow — contributing to off-tone, rambling replies. Extend the small
patterns to cover all sub-8B sizes (1b-7b) across :/-/_ separators and sync the
spec table.

tests/test_prompts.py

@@ -21,9 +21,15 @@ class TestModelSizeDetection:
         ("gemma:7b", True),
         ("phi3:3b", True),
         ("qwen2:7b", True),
+        # Sub-8B sizes that were previously misclassified as LARGE.
+        ("qwen3.5:4b", True),   # the deployed model that produced weak, off-tone replies
+        ("gemma2:2b", True),
+        ("model:5b", True),
+        ("model:6b", True),
         # Various separators
         ("model-3b-instruct", True),
         ("model_1b_chat", True),
+        ("model-4b-instruct", True),
         # Large models (should return LARGE)
         ("gpt-oss:20b", False),
         ("llama3.1:8b", False),