chore: gitignore *.mjjournal agent-local files

These are agent-local journal files that were untracked but not ignored,
so they showed up in `git status` and failed a local `reuse lint` run —
while CI passed, since an untracked file is absent from the checkout.
That discrepancy made the local lint useless as a pre-push gate.

Grouped with the existing agent-local exclusions (CLAUDE.md, .claude/,
Chat.txt). Local `reuse lint` now reports 7/7 and matches CI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: UnbreakableMJ

.gitignore

@@ -36,3 +36,4 @@ CODEX.md
 .aider*
 Chat.txt
 *.chat.txt
+*.mjjournal