chore: add .gitignore for Go build artifacts and tool output

Patterns added:
- *.test, *.prof, *.out, coverage.*: Go build/test artifacts
- .DS_Store, *.swp, .idea/, .vscode/: editor/OS files
- review.json: review tool output
- .omo/run-continuation/: omo session cache (currently 24 tracked files
  remain for history continuity; new ones will be ignored)
- .omo/evidence/: omo evidence dumps

Also untrack wal.test which was committed by mistake as a build binary.
The file still exists locally; only its tracking is removed.

.gitignore

@@ -0,0 +1,19 @@
+# Go build artifacts
+*.test
+*.prof
+*.memprof
+*.out
+coverage.out
+coverage.txt
+
+# Editor / OS
+.DS_Store
+*.swp
+*.swo
+.idea/
+.vscode/
+
+# Project tool artifacts (session caches, review output, evidence dumps)
+review.json
+.omo/run-continuation/
+.omo/evidence/