test(llm): lock in _should_think classifier; drop briefing think overrides

Adds 38 parametrized tests for the _should_think classifier covering the
explicit-override path, empty/whitespace content, short/medium/long length
boundaries, case-insensitive keyword matching, and a chatty-message negative
set. These pin the content-based semantics so future tweaks to the keyword
list or length thresholds surface regressions immediately instead of going
unnoticed behind subtle latency changes.

Also drops the `think=True` overrides from the briefing /discuss-article
and /discuss-topic entry points. With `"discuss"` added to _THINK_KEYWORDS,
those canned prompts trip the classifier naturally, so the overrides were
redundant — keeping a uniform "classifier is authoritative" rule makes the
code easier to reason about.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/generation_task.py

@@ -67,6 +67,7 @@ _THINK_KEYWORDS: tuple[str, ...] = (
     "review", "critique", "evaluate", "trade-off", "tradeoff", "trade off",
     "pros and cons", "step by step", "walk me through",
     "prove", "derive", "figure out", "work through",
+    "discuss",  # covers briefing /discuss-article + /discuss-topic entry points
 )
 
 # Messages shorter than this and without any think-keyword are treated as