fix: code review improvements for GH#870

Review findings addressed:
1. Fixed HasSyncBranchGitignoreFlags() - now correctly returns (hasAnyFlag,
   hasSkipWorktree) since skip-worktree takes precedence in git ls-files -v
2. Added interactions.jsonl to list of files to hide (was only issues.jsonl)
3. Added idempotency check - skips setting flags if already set (checks for S)
4. Made output conditional - only prints when flags actually changed
5. Fixed addToGitExclude() pattern matching - now uses exact line match
   instead of substring to prevent false positives
6. Refactored to use setGitIndexFlags() helper to reduce duplication

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/doctor/gitignore.go

@@ -310,7 +310,7 @@ func CheckSyncBranchGitignore() DoctorCheck {
 		}
 	}
 
-	hasAssumeUnchanged, hasSkipWorktree, err := fix.HasSyncBranchGitignoreFlags(cwd)
+	hasAnyFlag, _, err := fix.HasSyncBranchGitignoreFlags(cwd)
 	if err != nil {
 		return DoctorCheck{
 			Name:    "Sync Branch Gitignore",
@@ -320,7 +320,7 @@ func CheckSyncBranchGitignore() DoctorCheck {
 		}
 	}
 
-	if hasAssumeUnchanged || hasSkipWorktree {
+	if hasAnyFlag {
 		return DoctorCheck{
 			Name:    "Sync Branch Gitignore",
 			Status:  StatusOK,