chore: ignore /sylpheed.db — zq.py now defaults to it

The previous commit on this branch made `tools/zq.py` look for its database at
`<repo root>/sylpheed.db`, and its refusal message tells people to put one there.
Nothing ignored that path. The database is a build artefact of several hundred
MB, so a single `git add -A` would have committed it into a public repository's
history — the class of blob CONSOLIDATION.md's history-rewrite fork exists to
argue about.

Anchored (`/sylpheed.db`, plus DuckDB's `.wal`) so it covers only the default
location, not any file of that name elsewhere. Verified by creating the files:
both ignored, `git status` silent, and a `sylpheed.db` in a subdirectory is not
swallowed by the rule.

Found while writing the Phase 7 steps: moving the 336 MB database out of
`xenia-rs` before that clone is deleted is exactly how someone would put it here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -18,6 +18,13 @@ Thumbs.db
 # Local dev overrides
 .env
 
+# The static-analysis database `tools/zq.py` reads by default. A build artefact
+# of several hundred MB (`sylph-xexdb dis ... --db sylpheed.db`), and zq.py
+# now tells people to put it exactly here -- so it must never be committable.
+# One `git add -A` would otherwise put it in public history for good.
+/sylpheed.db
+/sylpheed.db.wal
+
 # Trunk build output
 dist/
 __pycache__/