chore(docs): untrack internal-only strategy and reference docs

docs/internal/ held internal scratch and business-strategy material
(architecture dumps under old/, plus drafts) that should not live on the
public repo. Remove the whole tree from tracking — local copies are kept
— and gitignore docs/internal/ so it can't be re-added by accident. Also
gitignore the internal smoke-findings tracker.

(History scrub of the already-pushed copies is a separate follow-up.)

.gitignore

@@ -96,3 +96,6 @@ panel/coverage/
 panel/tsconfig.tsbuildinfo
 panel/.env.local
 panel/.env.*.local
+# Internal-only: strategy/scratch/reference dumps — never publish
+docs/internal/
+SMOKE_FINDINGS_*.md