chore: remove tracked scratch files and ignore them

Posts.ts.tmp, tmp.txt, wget_out.txt, build logs, a debug HTML dump and a
generated markdown output were tracked but unreferenced. tmp_posts.json is
kept because scripts/extract-to-mdx.js still reads it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -61,3 +61,18 @@ backups/
 _at-mintel/
 local_build_*.log
 *.tar
+
+# scratch and generated artifacts (never commit)
+*.tmp
+*.log
+tmp.txt
+wget_out.txt
+build*.log
+public_test.html
+optimized_output.md
+
+# test, coverage and mutation output
+/coverage
+apps/web/coverage/
+apps/web/reports/
+.stryker-tmp/