feat: Respect BEADS_SYNC_BRANCH environment variable

Fixes daemon and bd sync to honor BEADS_SYNC_BRANCH environment variable
as documented in PROTECTED_BRANCHES.md for CI/CD temporary overrides.

Changes:
- Updated internal/syncbranch.Get() to prioritize env var over DB config
- Both daemon sync and bd sync CLI now use syncbranch.Get()
- Added comprehensive tests for env var override behavior
- Validates branch names using git-style rules

This enables CI/CD workflows to override sync branch per-job without
mutating database config.

Based on PR #364 by Charles P. Cross <cpdata@users.noreply.github.com>
Co-authored-by: Charles P. Cross <cpdata@users.noreply.github.com>

cmd/bd/sync.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/beads/internal/rpc"
+	"github.com/steveyegge/beads/internal/syncbranch"
 	"github.com/steveyegge/beads/internal/types"
 )
 
@@ -693,9 +694,9 @@ func getSyncBranch(ctx context.Context) (string, error) {
 		return "", fmt.Errorf("failed to initialize store: %w", err)
 	}
 
-	syncBranch, err := store.GetConfig(ctx, "sync.branch")
+	syncBranch, err := syncbranch.Get(ctx, store)
 	if err != nil {
-		return "", fmt.Errorf("failed to get sync.branch config: %w", err)
+		return "", fmt.Errorf("failed to get sync branch config: %w", err)
 	}
 
 	if syncBranch == "" {