fix: remove gitignore negations that override fork protection (#796)

The negation patterns (!issues.jsonl, !interactions.jsonl, etc.) in
.beads/.gitignore had higher precedence than the fork protection
exclusion in .git/info/exclude, effectively defeating fork protection.

Contributors could accidentally stage and commit upstream issue
databases because:
1. Fork protection added .beads/issues.jsonl to .git/info/exclude
2. .beads/.gitignore had !issues.jsonl which overrode the exclusion
3. .gitignore files have higher precedence than .git/info/exclude

The negation patterns were unnecessary anyway since no pattern in
.beads/.gitignore matches those files. Added a comment explaining
why negations should not be added.

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

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

cmd/bd/init_test.go

@@ -129,7 +129,7 @@ func TestInitCommand(t *testing.T) {
 					"beads.base.jsonl",
 					"beads.left.jsonl",
 					"beads.right.jsonl",
-					"!issues.jsonl",
+					"Do NOT add negation patterns", // Comment explaining fork protection
 				}
 				for _, pattern := range expectedPatterns {
 					if !strings.Contains(gitignoreStr, pattern) {