chore: gitignore /pipeline.db at repo root (phantom stale file)

A 0-byte pipeline.db sometimes appears at the repo root, left over
from old code paths. Current master correctly opens it at
.huskies/pipeline.db via project_root.join() in
server/src/startup/project.rs:280 — no relative-path opener exists.
This is purely defensive so any future regression doesn't sneak into
commits. Stops 1123 from being a coder task.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -15,6 +15,8 @@ _merge_parsed.json
 .huskies_port
 .huskies/bot.toml.bak
 .huskies/build_hash
+# Phantom 0-byte pipeline.db sometimes appears at repo root from old code; canonical DB lives at .huskies/pipeline.db
+/pipeline.db
 
 # Per-worktree planning file (written by coder agents, must never reach squash commits)
 PLAN.md