chore: remove .pid files from repo and gitignore them (#680)

These are runtime artifacts created by Conductor for worktree process
management. They should never be tracked in git.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -41,6 +41,9 @@ apps/web/test-results/
 # feature tracking
 _features/
 
+# runtime
+*.pid
+
 # platform specific
 *.dmg
 *.app

.worktree-backend.pid

@@ -1 +0,0 @@
-10462

.worktree-daemon.pid

@@ -1 +0,0 @@
-10589

.worktree-frontend.pid

@@ -1 +0,0 @@
-10473