Revert erroneous addition of .beads to agent .gitignore template (#891)

Gas town agents need to ignore working-file directories like .logs,
.runtime, and .claude, but Beads provides its own .gitignore handling
in `bd init`, creating a .beads/.gitignore file which ignores the
relevant Beads working files while allowing .beads/issues.jsonl
to be tracked correctly. PR #753 broke this, causing new polecats
to attempt to merge their changed .gitignore into the project repo,
ultimately breaking bd sync and causing issues to become untracked.

internal/rig/overlay.go

@@ -65,7 +65,6 @@ func EnsureGitignorePatterns(worktreePath string) error {
 	requiredPatterns := []string{
 		".runtime/",
 		".claude/",
-		".beads/",
 		".logs/",
 	}