Add a local benchmark for /tldr with a dashboard

`go test -tags bench ./internal/tldr -run TestBench` runs the real /tldr
pipeline against one or more Ollama models and prompt variants on a chat dump
from disk (text copied from Discord, "Author: message" lines, or a
DiscordChatExporter JSON export) and writes a self-contained HTML dashboard
plus a JSON of the results. Scoring uses an optional facts file (a fact
passes only when one bullet holds all its details) and automatic checks:
numbers not in the chat, copying the chat, an English intro, cut-off output.

TestFactsSkeleton proposes candidate facts from the chat to prune by hand.
Chat dumps and results stay local: bench-out/ and the private folders are
git-ignored. See internal/tldr/tldrbench/README.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/tldr/tldrbench/testdata/example-facts.json

@@ -0,0 +1,8 @@
+[
+  {"name": "Hosting 80 to 120", "all": ["120", "80"]},
+  {"name": "Quote by Monday", "all": ["orçamento|fornecedor", "segunda"]},
+  {"name": "Dinner Friday 20h", "all": ["jantar", "sexta", "20h"]},
+  {"name": "Dinner place and size", "all": ["cantinho", "9"]},
+  {"name": "Certificate renewed", "all": ["certificado|tls", "renov"]},
+  {"name": "14-day alert", "all": ["14 dias", "alerta|grafana"]}
+]