fix(sync): handle diverged histories with content-based merge (bd-3s8)

When multiple clones commit to beads-sync branch and histories diverge,
git merge would fail. This replaces git's commit-level merge with a
content-based merge that extracts JSONL from base/local/remote and
merges at the semantic level.

Key changes:
- Add divergence detection using git rev-list --left-right
- Extract JSONL content from specific commits for 3-way merge
- Reset to remote's history then commit merged content on top
- Pre-emptive fetch before commit to reduce divergence likelihood
- Deletions.jsonl merged by union (keeps all deletions)

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

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

cmd/bd/sync.go

@@ -384,7 +384,14 @@ Use --merge to merge the sync branch back to main branch.`,
 						os.Exit(1)
 					}
 					if pullResult.Pulled {
-						fmt.Printf("✓ Pulled from %s\n", syncBranchName)
+						if pullResult.Merged {
+							// bd-3s8 fix: divergent histories were merged at content level
+							fmt.Printf("✓ Merged divergent histories from %s\n", syncBranchName)
+						} else if pullResult.FastForwarded {
+							fmt.Printf("✓ Fast-forwarded from %s\n", syncBranchName)
+						} else {
+							fmt.Printf("✓ Pulled from %s\n", syncBranchName)
+						}
 					}
 					// JSONL is already copied to main repo by PullFromSyncBranch
 				} else {