chore: remove IMPLEMENTATION_REPORT.md from public repo

The file was a transient build / handoff doc and contained operator-
specific details (local absolute paths, named handoff target) that
should never have shipped to a public repo. Removed from the tracked
tree and added to .gitignore so future iterations stay local.

The file is preserved on disk for reference; .gitignore now matches
IMPLEMENTATION_REPORT*.md, HANDOFF*.md, and NOTES.local.md to prevent
similar slip-ins.

Note: this commit removes the file from HEAD only. The original
commit (f2dfcf0) remains in the repository's history. To purge it
from history requires a force-push history rewrite, which the
maintainer must perform explicitly if the leak risk warrants it.

Author: lktron00

.gitignore

@@ -37,3 +37,9 @@ app/data/*.sqlite3
 app/data/keys/
 bundles/
 *.zip
+
+# Internal handoff / build notes — never commit (may contain local paths,
+# operator names, deployment specifics that don't belong in a public repo).
+IMPLEMENTATION_REPORT*.md
+HANDOFF*.md
+NOTES.local.md